feat(tap-echo): Rust reference echo brain + test server (spec §2.3)
- start_echo_server(addr): in-process WS server for integration tests;
returns EchoHandle { shutdown, join, addr }.
- echo_one_connection: the per-connection echo loop — hello handshake,
audio_in → audio_out (same PCM), bye/session_end graceful close.
- Reuses rutster-tap's protocol types — the wire-types-reusable contract
test (spec §2.3).
- Stateless across reconnects (spec §5.3) — every hello starts fresh.
- Standalone binary: binds ws://127.0.0.1:8081, runs forever.
- 1 unit test exercises full hello/ack/audio_in/audio_out/bye over a
TCP loopback pair.
Spec ref: 2026-06-28-slice-2-agent-tap-design.md §2.3, §5.3.
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