Phase 4a: Add bounded local Unix IPC

[u9520107]

Summary

Add focusrited local Unix-socket IPC for Phase 4a.

• Versioned newline-delimited JSON v1 snapshot, command, event, and error messages.
• Daemon-owned socket with 0660 permissions and configurable --socket PATH.
• Commands route only through DeviceWorker; clients never access ALSA or USB.
• Bounded 64 KiB frames and outbound queues.
• Slow or malformed clients isolate from daemon and other clients.
• Authoritative snapshots, revisions, and instance_id support reconnect/resync.
• Add IPC protocol and Phase 4a documentation.

Safety

No hardware-control behavior added. Pi smoke used snapshots only: no command, profile apply, ALSA write, routing, or device-state mutation.

Verification

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

31 tests pass, including IPC framing, malformed input, live-socket protection, command ordering, external events, concurrent clients, queue overflow, and pipelined-frame handling.

Hardware action needed

None for merge. Touchscreen/UI validation belongs to Phase 4a MR 2.