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.
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