docs, flake: document tailored builds and name the flake features

flake.nix first: its native package passed a bare --no-default-features,
which used to mean "everything but web-ui" and now means *no providers
at all*. It names its set explicitly
(all-providers,opus,spectrum,notifications); the aarch64 cross build
keeps the full defaults and its staged wasm bundle.

docs/src/build-features.md: the feature table with what each one costs to
lose, why fs takes /crabidy, /orphans, queue persistence and scan with
it, the opus/libopus build note, two worked examples, what is
deliberately not gated, and check-features. Linked from SUMMARY.md, and
config.md now says the providers list can only offer what the binary was
built with. README gains a short pointer.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -45,6 +45,24 @@ cargo run -p cbd      # server + TUI in one process
 Or run the halves separately: `cargo run -p crabidy-server` and, in
 another terminal, `cargo run -p cbd-tui`.
 
+### Tailored builds
+
+Every provider — plus Opus decoding, the spectrum bars, the embedded web
+UI, and the TUI's desktop notifications — sits behind a Cargo feature,
+all on by default. Drop what you do not need and the dependencies go with
+it:
+
+```sh
+# a local-files appliance: no network providers, no web UI, no FFT
+cargo build --release -p crabidy-server --no-default-features --features fs,opus
+```
+
+`crabidy-server features` prints what a binary was built with. See
+[docs/src/build-features.md](docs/src/build-features.md) for the full
+table (and what `fs` takes with it), and
+[architecture/build-features.md](architecture/build-features.md) for the
+design.
+
 ## Configuration
 
 All configuration lives in `~/.config/crabidy/` (the platform config

docs/src/SUMMARY.md

@@ -17,4 +17,5 @@
   - [The cbd bundle](./clients/cbd.md)
   - [Command line](./clients/cli.md)
 - [Configuration](./config.md)
+- [Tailored builds](./build-features.md)
 - [Roles and authorization](./auth.md)

docs/src/build-features.md

@@ -0,0 +1,135 @@
+# Tailored builds
+
+<!-- toc -->
+
+Every provider, Opus decoding, the spectrum bars, the embedded web UI and
+the TUI's desktop notifications sit behind a Cargo **feature**. All of them
+are **on by default**, so a plain `cargo build` gives you the full player.
+Turning some off gives you a smaller binary that pulls fewer dependencies —
+useful for a single-purpose box (a Raspberry Pi playing a local flac
+collection) or a build environment you want to keep lean.
+
+This is the *compile-time* half of provider selection. The runtime half —
+the `providers` list in `crabidy-server.toml` — still works exactly as
+before; see [Configuration](./config.md#enabling-and-disabling-providers).
+The two compose in one direction: **a provider that is not compiled in
+cannot be enabled in the config**, and if you name one anyway the server
+logs a warning and carries on.
+
+## What this build has
+
+```sh
+crabidy-server features   # or: cbd features
+```
+
+prints one feature per line — the providers it can mount, then the extras.
+The same list goes into the server's startup log, so a support question
+("why is `/tidal` missing?") is answerable from the binary and its log.
+
+## The features
+
+| Feature      | Turning it off drops                                        |
+| ------------ | ----------------------------------------------------------- |
+| `tidal`      | the `/tidal` provider (`tidaldy`)                           |
+| `youtube`    | the `/youtube` provider (`ytdy`, and with it `rustypipe`)   |
+| `fyyd`       | the `/fyyd` podcast provider                                |
+| `abs`        | the `/abs` audiobookshelf provider                          |
+| `soundcloud` | the `/soundcloud` provider                                  |
+| `jamendo`    | the `/jamendo` provider                                     |
+| `fs`         | local files **and persistent state** — see below            |
+| `opus`       | Ogg-Opus decoding (`symphonia` + a bundled libopus C build) |
+| `spectrum`   | the server-side FFT feeding clients' spectrum bars          |
+| `web-ui`     | the embedded web client (`tonic-web` + the wasm bundle)     |
+
+Plus two conveniences: `all-providers` enables the seven provider features
+at once, and `cbd` (the bundle) mirrors every feature above and adds
+`notifications` for the TUI's desktop "now playing" popups (`notify-rust`,
+which on Linux pulls a D-Bus stack).
+
+### `fs` is more than `/fs`
+
+The `/fs` provider, the content store, and everything built on it are all
+written against the same crate, so they share one feature. With `fs` off you
+lose:
+
+- the `/fs` mount (your local music folder);
+- `/crabidy` — saved queues, bookmarks (`w`) and captures (`W`);
+- `/orphans`, which is a view over the store;
+- **queue persistence**: the queue lives in memory, so a restart starts
+  empty;
+- the `scan` command (it fails with a message naming the feature);
+- captured-track markers in library listings.
+
+The capture and save-queue RPCs answer `Unimplemented` on such a server, so
+clients report an ordinary error instead of hanging.
+
+If you want `/fs` but not `/crabidy`, keep the feature and prune the
+`providers` list at runtime instead — that is what it is for.
+
+### `opus` and the libopus build
+
+`symphonia` (and so rodio) has no Opus decoder, so Ogg-Opus files are
+decoded through a bundled libopus, which needs `cmake` at build time.
+Turning `opus` off removes both the crates and that build requirement.
+
+The feature also decides whether `scan` treats `.opus` files as playable at
+all, so a build that cannot decode Opus will not index Opus files either. An
+Opus file that reaches such a build fails to decode with a message naming
+the missing feature and is skipped, exactly like any unplayable file.
+
+### `spectrum` and `web-ui`
+
+`spectrum` only affects the *server*: with it off no frames are computed or
+broadcast, and clients simply show no bars — nothing else changes, and no
+client needs rebuilding. `web-ui` drops the embedded browser client; the
+gRPC service on port 50051 still serves `cbd-tui` and the CLI.
+
+## Examples
+
+A local-files appliance — no network providers, no web UI, no FFT:
+
+```sh
+cargo build --release -p crabidy-server \
+  --no-default-features --features fs,opus
+```
+
+A streaming box with the browser client and the bars, no local library:
+
+```sh
+cargo build --release -p crabidy-server \
+  --no-default-features --features tidal,web-ui,opus,spectrum
+```
+
+The bundle, tailored the same way (its features forward to the server):
+
+```sh
+cargo build --release -p cbd --no-default-features --features fs,opus
+```
+
+A terminal client with no D-Bus dependency:
+
+```sh
+cargo build --release -p cbd-tui --no-default-features
+```
+
+`--no-default-features` on its own is legal and compiles: you get a server
+that starts, serves an empty library and plays nothing. It is the base case
+the feature matrix checks, not a useful deployment.
+
+## What is *not* behind a feature
+
+- **Authorization.** `[auth]` and its password hashing always ship. A build
+  that ignored configured role hashes would silently run an
+  intended-to-be-locked server open — a fail-open hole not worth a small
+  dependency.
+- **Audio output.** The server is the player; a server without audio has no
+  purpose.
+- **HLS streaming and the spectrum tap.** They bring no dependencies of
+  their own, so gating them would add build complexity and save nothing.
+
+## Checking combinations
+
+`devenv shell -- check-features` runs the curated matrix — defaults, no
+features, each provider alone, each extra dropped, both examples above, and
+the client crates — and requires every one to be clippy-clean. Run it after
+changing anything that sits behind a feature.

docs/src/config.md

@@ -47,6 +47,13 @@ left unread. Deleting the whole `providers` line re-enables everything (the
 same as a fresh install with no file). Because `orphans` is a view over the
 store, disabling `crabidy` disables `orphans` too.
 
+The list can only offer what the binary was **built** with: providers are
+also selectable at compile time (see [Tailored
+builds](./build-features.md)). The default list written on first start
+therefore names only the providers this build has, and if you add one it
+does not have, the server logs a warning at startup and ignores it. Run
+`crabidy-server features` to see what a binary contains.
+
 ## Client config: `cbd-tui.toml` and `cbd.toml`
 
 `cbd-tui` (the standalone terminal client) reads `cbd-tui.toml`; `cbd`

flake.nix

@@ -54,11 +54,18 @@
         };
 
         # Only the real binary crates; never build cbd-web (a wasm crate) for a
-        # host or aarch64 target. Headless: --no-default-features drops the
+        # host or aarch64 target.
-        # embedded web UI. The aarch64 server (below) re-enables it and stages
-        # the wasm bundle.
         workspaceBins = "-p crabidy-server -p cbd -p cbd-tui";
 
+        # Headless: everything except the embedded web UI. Spelled out rather
+        # than "--no-default-features" alone, which since
+        # architecture/build-features.md means *no providers at all* — the
+        # features must be named or this package ships an empty server. The
+        # aarch64 server (below) takes the full defaults and stages the wasm
+        # bundle. `notifications` is cbd-tui's own default and needs no
+        # mention here.
+        headlessFeatures = "all-providers,opus,spectrum,notifications";
+
         # --- Web UI (wasm) bundle ---------------------------------------------
         # cbd-web is compiled to wasm by trunk; the output dist/ is what
         # crabidy-server's build.rs embeds (feature `web-ui`). The bundle is
@@ -130,7 +137,7 @@
         nativeArgs = {
           inherit src;
           strictDeps = true;
-          cargoExtraArgs = "--locked --no-default-features ${workspaceBins}";
+          cargoExtraArgs = "--locked --no-default-features --features ${headlessFeatures} ${workspaceBins}";
           # protoc + pkg-config are build-host tools (run during the build);
           # with strictDeps they must be nativeBuildInputs or alsa-sys cannot
           # find pkg-config. alsa-lib is the actual linked library.

plan/build-features.md

@@ -13,24 +13,24 @@ devenv shell -- bash -lc 'CARGO_TARGET_DIR="$(pwd)/target-claude" cargo … '
 
 ## A — Mount registry (behaviour-preserving refactor)
 
-- [ ] **A1 — `Mount` + registry types.** In `provider.rs`: `struct Mount {
+- [x] **A1 — `Mount` + registry types.** In `provider.rs`: `struct Mount {
   root: &'static str, name: &'static str, client: Arc<dyn ProviderClient> }`,
   `Mount::new(root, name, Arc<impl ProviderClient>) -> Mount` (the `Arc`
   coerces at the call site), and `Mount::owns(&self, path) -> bool` matching
   `path == root || path.starts_with("<root>/")` — so `/fsx` is not `/fs`.
   *Verifies:* `mount_owns_its_root_and_children_only` (G10).
-- [ ] **A2 — `ProviderOrchestrator` holds `mounts: Vec<Mount>`.** Replace the
+- [x] **A2 — `ProviderOrchestrator` holds `mounts: Vec<Mount>`.** Replace the
   nine `*_client` fields (keep `crabidy_store`). Add
   `from_mounts(Vec<Mount>) -> Self` (sorts the mounts crabidy-first /
   orphans-last / rest alphabetical, and creates the bounded channel), used by
   `build` and the tests. *Verifies:* `root_lists_mounted_providers_in_order`
   (G12).
-- [ ] **A3 — One dispatch helper.** `owner(&self, path) -> Option<&dyn
+- [x] **A3 — One dispatch helper.** `owner(&self, path) -> Option<&dyn
   ProviderClient>` plus `owner_or(&self, path, err)` that warns with the same
   message and returns the typed error. Delete the nine `*_owns` functions and
   the nine `*_provider` accessors. *Verifies:* `unowned_paths_are_typed_errors`
   (G11).
-- [ ] **A4 — Rewrite the eight dispatch methods** (`is_track_path`,
+- [x] **A4 — Rewrite the eight dispatch methods** (`is_track_path`,
   `get_urls_for_track`, `get_metadata_for_track`, `get_lib_node`,
   `create_lib_node`, `rename_lib_node`, `delete_lib_node`,
   `resolve_tracks_into`) as a single lookup each, preserving: the synthetic
@@ -39,35 +39,35 @@ devenv shell -- bash -lc 'CARGO_TARGET_DIR="$(pwd)/target-claude" cargo … '
   `warn!`s. *Verifies:* `dispatch_reaches_the_owning_mount`,
   `overridden_resolve_tracks_into_is_dispatched`,
   `no_mounts_serves_an_empty_root` (G10, G11, G13).
-- [ ] **A5 — `get_lib_root` maps the sorted mounts** to `LibraryNodeChild::new`
+- [x] **A5 — `get_lib_root` maps the sorted mounts** to `LibraryNodeChild::new`
   (no per-provider block, no re-sort). *Verifies:*
   `root_lists_mounted_providers_in_order` (G12).
-- [ ] **A6 — `build()` registers mounts.** Each provider's init block stays as
+- [x] **A6 — `build()` registers mounts.** Each provider's init block stays as
   is; on success it pushes a `Mount` instead of assigning a field. `/crabidy`
   and `/orphans` push after the store exists. *Verifies:* full test suite +
   `cargo clippy -- -D warnings` (G2).
-- [ ] **A7 — Commit A.** `cargo test -p crabidy-server`, clippy, fmt all clean;
+- [x] **A7 — Commit A.** `cargo test -p crabidy-server`, clippy, fmt all clean;
   no `Cargo.toml` change in this commit (G1/G2 hold trivially).
 
 ## B — The features
 
-- [ ] **B1 — Manifests** (done in api-design; re-verify): `crabidy-server`
+- [x] **B1 — Manifests** (done in api-design; re-verify): `crabidy-server`
   (`default = all-providers + opus + spectrum + web-ui`, gated deps optional),
   `audio-player` (`opus`), `cbd-tui` (`notifications`), `cbd` (pass-through of
   all of them). *Verifies:* G4, G5; `cargo tree -p crabidy-server` at defaults
   matches the pre-change tree (G1).
-- [ ] **B2 — `settings`: `BUILT_IN_PROVIDERS`, `provider_enabled` bounded by
+- [x] **B2 — `settings`: `BUILT_IN_PROVIDERS`, `provider_enabled` bounded by
   it, `unavailable_providers()`, `ensure_default` writing only built-ins.**
   Replace the `todo!()`. *Verifies:*
   `an_absent_providers_key_enables_every_built_in_provider`,
   `unavailable_providers_reports_built_out_and_unknown_names`,
   `ensure_default_lists_only_built_in_providers`,
   `a_built_out_provider_cannot_be_enabled` (G4, G25, G26).
-- [ ] **B3 — Per-provider `#[cfg]` in `build()` only.** Wrap each provider's
+- [x] **B3 — Per-provider `#[cfg]` in `build()` only.** Wrap each provider's
   init+registration block in `#[cfg(feature = "…")]`; the `enabled.*` toggle
   read stays inside the block. Provider `use`/imports move to the same gate.
   *Verifies:* the matrix (B12) compiles each provider alone; G9 by reading.
-- [ ] **B4 — `fs`: gate the store half.** `#[cfg(feature = "fs")]` on the
+- [x] **B4 — `fs`: gate the store half.** `#[cfg(feature = "fs")]` on the
   `crabidy_store`, `capture`, and `orphans` modules (`lib.rs`), on
   `ProviderOrchestrator::crabidy_store`/its field, on `Playback`'s store field
   and `restore_current` and the persister, and on the `/fs`, `/crabidy`,
@@ -75,66 +75,66 @@ devenv shell -- bash -lc 'CARGO_TARGET_DIR="$(pwd)/target-claude" cargo … '
   parameter becomes `#[cfg]`-free by threading `Option<Arc<CrabidyStore>>`
   only under the feature (adjust the two call sites in `lib.rs` and the
   playback tests). *Verifies:* G14, G17; `--no-default-features` build (B12).
-- [ ] **B5 — `fs`: RPC degradation.** Without `fs`, `CaptureLibraryNode` and
+- [x] **B5 — `fs`: RPC degradation.** Without `fs`, `CaptureLibraryNode` and
   `SaveQueue` return `Status::unimplemented` naming the feature; with `fs`,
   unchanged. No panic, no hang. *Verifies:* G15 by reading + a
   `--no-default-features` smoke run.
-- [ ] **B6 — `fs`: the `scan` command.** Gate `AUDIO_EXTENSIONS`,
+- [x] **B6 — `fs`: the `scan` command.** Gate `AUDIO_EXTENSIONS`,
   `is_audio_file`, `scan`, `scan_dir`, `scan_file` and their tests behind
   `fs`; without it the dispatcher in `crabidy-server/src/main.rs` and
   `cbd/src/main.rs` prints "this binary was built without the `fs` feature"
   and exits non-zero. The clap surface does not change. *Verifies:* G16, G9.
-- [ ] **B7 — `opus` in `audio-player`.** Gate `mod opus_source` and the
+- [x] **B7 — `opus` in `audio-player`.** Gate `mod opus_source` and the
   `is_ogg_opus` sniff in `player_engine::build_source`; without the feature an
   Ogg-Opus header produces `Err` naming the missing feature (the existing
   "failed to decode" path), never a panic. *Verifies:* G18, G19 (read + the
   `opus`-off matrix entry); `opus_source`'s own tests stay under the feature.
-- [ ] **B8 — `opus` in `scan`.** `AUDIO_EXTENSIONS` includes `"opus"` only
+- [x] **B8 — `opus` in `scan`.** `AUDIO_EXTENSIONS` includes `"opus"` only
   under the feature. *Verifies:* `opus_is_scannable_only_with_the_decoder`,
   `scan_walks_past_opus_files_it_cannot_play` (G20).
-- [ ] **B9 — `spectrum`.** Gate `mod spectrum`, `spawn_spectrum_task`, and its
+- [x] **B9 — `spectrum`.** Gate `mod spectrum`, `spawn_spectrum_task`, and its
   call; `realfft` optional. `Player::spectrum_tap()` unchanged. *Verifies:*
   G21; the `spectrum`-off matrix entry.
-- [ ] **B10 — `notifications` in `cbd-tui`.** Gate the `notify_rust` import and
+- [x] **B10 — `notifications` in `cbd-tui`.** Gate the `notify_rust` import and
   the notify call in `app/now_playing.rs`; the config key stays accepted so an
   existing `cbd-tui.toml` still parses. *Verifies:* G23; `cbd-tui
   --no-default-features` build + `cargo test -p cbd-tui`.
-- [ ] **B11 — `features` command.** `cli::build_features()` +
+- [x] **B11 — `features` command.** `cli::build_features()` +
   `cli::features()` (replace the `todo!()`s), wired into
   `ServerCommand::Features` and `CbdCommand::Features`; plus one `info!` line
   at startup listing the
   build, and one `warn!` per `unavailable_providers()` entry. *Verifies:*
   `build_features_lists_providers_then_extras` (G24, G25).
-- [ ] **B12 — The build matrix as a devenv script.** `check-features` runs
+- [x] **B12 — The build matrix as a devenv script.** `check-features` runs
   `cargo clippy -- -D warnings` (and `cargo test` where meaningful) over:
   defaults · `--no-default-features` · each of the seven providers alone ·
   defaults-minus-`opus` · minus-`spectrum` · minus-`web-ui` · `-p cbd-tui
   --no-default-features` · `-p audio-player --no-default-features`. Add
   `cargo-hack` to devenv for ad-hoc deeper sweeps. *Verifies:* G6, G7.
-- [ ] **B13 — Commit B** once the whole matrix is green and no
+- [x] **B13 — Commit B** once the whole matrix is green and no
   `todo!()`/`unimplemented!()` from the stub stage remains (G28).
 
 ## C — Packaging and docs
 
-- [ ] **C1 — `flake.nix`.** Native build: replace the bare
+- [x] **C1 — `flake.nix`.** Native build: replace the bare
   `--no-default-features` with an explicit
   `--no-default-features --features all-providers,opus,spectrum`; leave the
   aarch64 cross build on defaults (`web-ui` on). *Verifies:* G8;
   `nix build .#crabidy` if the sandbox allows, otherwise read + the equivalent
   cargo invocation.
-- [ ] **C2 — `docs/src/build-features.md`** — the feature table, what each one
+- [x] **C2 — `docs/src/build-features.md`** — the feature table, what each one
   costs to lose, the two worked examples (local-files appliance:
   `--no-default-features --features fs,opus`; streaming box:
   `--no-default-features --features tidal,web-ui,opus,spectrum`), the
   compile-time vs runtime distinction, and the `features` command. Linked from
   `SUMMARY.md`; `docs/src/config.md` cross-references it from the `providers`
   key. *Verifies:* G27 + `mdbook build`.
-- [ ] **C3 — `README.md`** gains a short "tailored builds" pointer.
+- [x] **C3 — `README.md`** gains a short "tailored builds" pointer.
   *Verifies:* G27.
-- [ ] **C4 — `plan/summary.md`** entry describing what shipped, the decisions
+- [x] **C4 — `plan/summary.md`** entry describing what shipped, the decisions
   taken autonomously, and what stayed out (auth, HLS, spectrum tap — with the
   reason). *Verifies:* dev-flow convention.
-- [ ] **C5 — Commit C.**
+- [x] **C5 — Commit C.**
 
 ## Deferred / not done (recorded, not silently dropped)
 

plan/summary.md

@@ -1201,3 +1201,81 @@ covering enter/step/jump/back-sweep/exit/Esc/non-move-exit/node-focus-exit/
 is_queable/filter); clippy `-D warnings` and fmt clean; markdownlint clean. Not
 exercised: live in-terminal keypresses (no TTY here) — the pure dispatch/table
 logic is fully unit-tested and the render path compiles.
+
+## build features (2026-07-25)
+
+Guarded each provider, Opus decoding, the spectrum, and desktop
+notifications behind Cargo features (all default-on) so people can build a
+non-bloated binary, with the dependency graph aligned to the flags. Ran the
+full dev-flow: `architecture/build-features.md` (D1–D11),
+`quality/build-features.md` (G1–G30), `plan/build-features.md` (A/B/C).
+Three commits.
+
+**A — mount registry** (`crabidy-server/src/provider.rs`). The orchestrator
+held nine `Option<Arc<ConcreteClient>>` fields and repeated the same
+`if …_owns(path)` chain across eight `ProviderClient` methods.
+`ProviderClient` is dyn-compatible (`init` carries `Self: Sized`), so it now
+holds `mounts: Vec<Mount>` of `Arc<dyn ProviderClient>` and every method is
+one `owner(path)` lookup — ~600 lines of repetition gone, and a provider is
+named in exactly one place, which is what made the feature work tractable.
+Ordering (crabidy first, orphans last, rest alphabetical) moved to
+`from_mounts`; the five config-file providers share a `mount_from_config`
+helper. Behaviour-preserving, landed on its own with 6 new unit tests
+(ownership boundaries, dispatch, typed errors for unowned paths, ordering,
+the empty registry, dyn dispatch of an overridden `resolve_tracks_into`).
+
+**B — the features.** `crabidy-server`: `tidal`, `youtube`, `fyyd`, `abs`,
+`soundcloud`, `jamendo`, `fs`, `opus`, `spectrum`, `web-ui`, plus
+`all-providers`; `cbd` forwards all of them and adds `notifications`;
+`cbd-tui` owns `notifications`; `audio-player` owns `opus`. Gated deps are
+`optional = true`, and the three feature-carrying local crates get
+`default-features = false` in `[workspace.dependencies]` (a member cannot
+drop an inherited default).
+
+**Answers to the two questions in the request:** `scan` *did* already treat
+`.opus` as playable; that entry is now conditional on `opus`, so a build
+that cannot decode Opus will not index it either. And `fs` off does take
+`/crabidy` and `/orphans` with it.
+
+Decisions taken autonomously (dev-flow, no questions):
+
+- **A feature must pay for itself in dependencies.** That is why `crabidy`
+  and `orphans` are *not* separate features (no dependency of their own —
+  `crabidy_store`/`capture`/`orphans` are written against `fsdy`), and why
+  `hls.rs`, `windowed_http.rs`, and `spectrum_tap.rs` stay unconditional.
+  Finer-grained control already exists at runtime in the `providers` list.
+- **`fs` is one coherent unit**: `/fs`, the content store, `/crabidy`,
+  `/orphans`, bookmarks/captures, queue persistence, and `scan`. Without it
+  `CaptureLibraryNode`/`SaveQueue` answer `Unimplemented` and `scan` reports
+  the missing feature — no panics, no hangs.
+- **`[auth]`/argon2 is deliberately not gated** (D9): a build that ignored
+  configured role hashes would run an intended-to-be-locked server open.
+  Refused as a fail-open hole for a small dependency.
+- **The compile-time set bounds the runtime one**: `BUILT_IN_PROVIDERS`
+  filters `provider_enabled`, the auto-written default config lists only
+  built-ins, and a `providers` entry this binary lacks logs one warning
+  (fail-open, unlike auth). `crabidy-server features` / `cbd features` print
+  the compiled set and startup logs it.
+- **An internal `_any-provider` marker feature** (every provider enables it)
+  expresses "is any provider compiled in?", which Cargo features cannot;
+  it keeps the legal-but-degenerate zero-provider build warning-free without
+  blanket `allow`s.
+- **`flake.nix` had to change in the same breath**: its native package
+  passed a bare `--no-default-features` (meaning "all but `web-ui`"), which
+  after this work would have shipped a provider-*less* binary. Now
+  `--features all-providers,opus,spectrum,notifications`.
+
+Verification: `devenv shell -- check-features` (new script) runs the curated
+matrix — defaults, `--no-default-features`, each of the seven providers
+alone, each extra dropped, the two documented examples, `audio-player`,
+`cbd-tui`, `cbd` — all clippy-clean under `-D warnings`, with tests at both
+extremes. Full default suite green (88 server + 90 tui + the rest). **G1
+verified mechanically**: the default `cargo tree -p crabidy-server` set is
+identical to the pre-change tree. Confirmed by inspection of `cargo tree`
+that `--no-default-features` drops every gated crate, `opus`-off drops
+`opusic-sys`/`symphonia-adapter-libopus`, and `cbd-tui
+--no-default-features` drops `notify-rust`. Smoke-ran both the
+provider-less and the `fs,opus` binaries under isolated XDG dirs: startup
+logs the feature list, writes a default config listing only built-ins, and
+warns once per unavailable name. Not exercised: `nix build` (not run here)
+and playback of an Opus file in an `opus`-less build (no audio device).