feat(tap): TapAudioPipe + TapMetrics (spec §4.1)

- TapMetrics: atomic counters (drop + observe per slice-1 §3.8).
- TapAudioPipe: AudioSource + AudioSink impl over mpsc + VecDeque ring.
  Drop-oldest on overflow (lowest-latency-correct); silence on underflow.
- TAP_PLAYOUT_FRAMES = 5 (100ms @ 20ms/frame; tunable constant per spec §4.1).
- 6 unit tests cover underflow, overflow drops-oldest, sink-full drops,
  disconnected engine, frame round-trip, empty ring.

Spec ref: 2026-06-28-slice-2-agent-tap-design.md §4.1.

crates/rutster-tap/src/lib.rs

@@ -27,18 +27,22 @@
 //! — that would invert the canonical-home of `PcmFrame` and pull the
 //! loopback peer into the tap story.
 
+pub mod metrics;
 pub mod protocol;
+pub mod tap_audio_pipe;
 
 // Re-export PcmFrame so `rutster-tap` consumers (the binary, the echo brain,
 // future brains) get it from one canonical home (spec §3.1).
 pub use rutster_media::PcmFrame;
 
+pub use metrics::{MetricsSnapshot, TapMetrics, TAP_PLAYOUT_FRAMES};
 pub use protocol::{
     decode_envelope, decode_pcm, encode_audio_in, encode_audio_out, encode_bye, encode_error,
     encode_hello, encode_pcm, encode_session_end, AudioPayload, DecodedFrame, DecodedPayload,
     Envelope, ErrorPayload, FrameKind, HelloPayload, Payload, ReasonPayload, SessionEndPayload,
     TapProtoError, PROTOCOL_VERSION, SAMPLES_PER_FRAME as WIRE_SAMPLES_PER_FRAME,
 };
+pub use tap_audio_pipe::TapAudioPipe;
 
 #[cfg(test)]
 mod tests {