feat(desktop): add local community tracer (PR 1A/12)

[loganj]

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Summary

• Adds the community-only SQLite persistence slice and desktop local-relay lifecycle for local mode, behind the default-off local_communities experiment gate.
• Keeps the bundled SingleNode sidecar private: application, health, and metrics listeners bind loopback only. It also uses atomic durable service-key persistence plus distinct-port allocation, bind-conflict retry, and bounded stderr diagnostics.
• Makes SQLite add_member match the public Postgres contract transactionally, including key validation, private-channel admission, role/last-owner restrictions, and original-inviter preservation across active re-add and soft-delete reactivation.
• This is stacked on refactor(relay): backend seams prep for local mode (PR 0/12) #5506 (brother-darryl/local-mode-pr0-seams) and retargets to main after that PR merges.

Fix-round follow-up

• Confined all SingleNode listeners to loopback.
• Added the complete transactional SQLite membership invariant family and provenance-parity regressions.
• Replaced direct service-key writes with the atomic owner-only persistence primitive.
• Hardened sidecar port allocation, retry, and diagnostics.
• Cleared push checks by using explicit stderr if/else handling and moving the local auth-tag backfill to agent_auth.rs, avoiding an agents.rs file-size-ratchet exception.

Integration debt

Conflicts with current main in WelcomeSetup.tsx and onboarding.spec.ts from #5310 onboarding polish. Deliberately deferred: resolve in the rebase-onto-main at retarget time after #5506 merges, with delta review of the resolutions.

Related issue

N/A — stacked follow-up to #5506.

Testing

• Hermit pre-push checks passed with hooks enabled: desktop check/typecheck/test, mobile test, Rust tests, and desktop Tauri checks.
• cargo test -p buzz-db --lib: 97 passed, 154 ignored.
• Relay baseline/head comparison had identical known failures; this host's PostgreSQL was down, so CI is authoritative for PostgreSQL coverage.