feat: rewrite desktop audio I/O with lock-free ring buffers

- Replace Mutex-based CPAL callbacks with atomic SPSC ring buffers
- Proper async send/recv loops (no block_on), 20ms playout tick
- Add signal task for RoomUpdate presence display
- Add --alias, --raw-room flags and key persistence (~/.wzp/identity)
- Add SetAlias signal variant and relay-side handling
- Graceful Ctrl+C shutdown with force-quit on second press

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-client/src/lib.rs

@@ -8,6 +8,8 @@
 
 #[cfg(feature = "audio")]
 pub mod audio_io;
+#[cfg(feature = "audio")]
+pub mod audio_ring;
 pub mod bench;
 pub mod call;
 pub mod drift_test;