feat(tap): versioned JSON event wire protocol (spec §3)
- protocol.rs: Envelope + FrameKind + Payload types, serde-derived.
- Explicit little-endian PCM codec (to_le_bytes / from_le_bytes) —
spec §3, §9. No host-endian silent hazard.
- encode_* helpers for hello, audio_in, audio_out, session_end, bye, error.
- decode_envelope validates protocol version + samples count; unknown
type values decode to DecodedPayload::Unknown (log + count + drop
per spec §3.4).
- 10 unit tests cover round-trips, LE byte order, sample-count
mismatch, unknown-type drop, and version mismatch.
Spec ref: 2026-06-28-slice-2-agent-tap-design.md §3.
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