feat(ws): loading-indicator audio loop on a dedicated LiveKit track

[tigranbs]

Summary

Adds a loading-indicator audio capability to the WebSocket API — a short, client-supplied clip that loops into the LiveKit room while the calling application is busy ("thinking"). It is the audio equivalent of a spinner: the human participant hears "still working" instead of ambiguous silence.

The loading audio plays on its own dedicated, second published LiveKit track (loading-audio), fully independent of the speech track (tts-audio), so it never interferes with the STT/TTS pipeline.

What changed

• Config — the config message gains an optional loading_audio object: base64-encoded WAV or raw 16-bit PCM, with optional format, sample_rate, channels, and volume (0.0–1.0). The clip is decoded and validated once per session; a decode failure is non-fatal (the session continues without the feature).
• Commands — two new fire-and-forget WebSocket commands:
  • loading_start — begins the seamless loop.
  • loading_stop — stops the loop with a short linear fade-out.
• Dedicated track — a second NativeAudioSource + LocalAudioTrack is published at the clip's own sample rate, so no resampling is needed. The loop captures frames directly on its own source and never touches the TTS operation queue, audio queue, or generation state.
• Playback — seamless cursor-based looping with wrap-around; ~30 ms fade-out on stop; volume applied once at decode time.
• Lifecycle — track publish on connect, re-publish on reconnect, and full teardown (cancel → await → abort backstop) on disconnect.

The loop is controlled exclusively by loading_start / loading_stop. speak and clear are unaffected — applications send loading_stop before speak if they don't want overlap.

Validation

• Input is validated: base64, size cap, 16-bit-PCM only, sample-rate range, channel count, duration bounds, and frame alignment — each with a clear client-facing error.
• No new third-party dependencies (hound, base64, tokio-util were already declared).

Testing

• Unit tests for decode/validation, loop mechanics (cursor wrap, fade-out), volume scaling, and message (de)serialization.
• LiveKit client tests for track setup, start/stop idempotency, disconnect teardown, rapid start/stop stress, and drop-backstop cancellation.
• All CI checks pass locally: cargo fmt --all -- --check, cargo clippy --all-targets --all-features -- -D warnings, cargo build --locked --all-features, cargo test --locked --all-features (105 tests passing).

Docs

docs/websocket.md, docs/livekit_integration.md, docs/api-reference.md, and the generated docs/openapi.yaml are updated; CLAUDE.md notes the new commands.