Replace the "any hash configured => every RPC needs credentials" switch
with a top-down model: a request with no credentials is granted the most
privileged role whose password is not set, and each password lowers that
floor. Nothing guarded -> owner (the open default); guard owner ->
anonymous is queue-owner; guard owner+queue_owner -> queue-appender;
guard all three -> credentials required for everything. A credential
still elevates a caller to its role; a present-but-wrong credential is
denied, never silently downgraded to the anonymous role.
Because the anonymous role is always the highest unguarded one, guarding
a lower role while a higher one is open is meaningless. Valid guarded
sets are prefixes of [owner, queue_owner, queue_appender];
AuthSettings::validate rejects any other order, load aborts startup on
it (fail-closed), and `guard` refuses to write it.
Docs (architecture, quality, mdbook, README) updated to the new model.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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