Records the current design in architecture/overview.md: crate topology,
the three concurrency domains and the channels between them, the
filesystem-like path addressing scheme, the span-carrying message
envelope used for tracing, and the Tidal device-flow auth.
Captures the trade-offs behind the load-bearing choices (paths vs opaque
ids, blocking engine thread, lossy broadcast) and the known gaps, so
later changes can see what was traded away.
Adds d2 to devenv for rendering the embedded diagrams.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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