feat: text-to-music mode (generate from prompt alone, no input audio)

[leszko]

Summary

Adds a text-to-music mode: generate music in realtime from the text prompt alone, with no input audio — selectable at session start and switchable mid-session, in both the backend and the web app.

ACE-Step's trained "no reference audio" signal is the checkpoint's canonical silence latent (the model forward uses it to simulate text2music mode), so a text-only session is a normal streaming session whose source latent and structure context are the silence latent, diffusing from pure noise at denoise=1.0. No model or pipeline changes — only session construction and wiring.

Changes

Contract (registry-first):

• SessionConfig gains text2music: bool and text2music_duration_s: float = 60.0 — auto-projected into GET /api/protocol and the generated TS types.
• swap_source command gains a text2music field (no binary PCM frame, mirrors use_server_source).
• wireContract.gen.ts regenerated.

Engine (acestep/streaming):

• source.py: text2music_waveform() — silent placeholder that seeds the playback ring (the user hears generated slices stream in over silence) — and resolve_text2music_source() — canonical-silence PreparedSource via EmptyLatent plus fixed 120 BPM / C major / 4 conditioning defaults. Skips VAE encode, semantic extract, librosa beat-tracking (returns 0 BPM on silence, which would poison the text conditioning), CNN key detection, and stem extraction.
• session.py: create() and the swap path branch on the flag; swap_source() accepts text2music.

Transport + SDK:

• ws_adapter.py synthesizes the silent source server-side for both the init handshake and t2m swaps; no audio frame crosses the wire.
• web/sdk/protocol.ts: connect() skips the PCM frame when config.text2music is set; new sendSwapTextToMusic().

Web app:

• New client-side sentinel source (web/lib/text2music.ts) so every picker surface works unchanged: pinned TEXT TO MUSIC sleeve in the crate fan, pinned entry in the CORE-tab track picker, option in the lite select.
• useStartSession / useFixtureSwap translate the sentinel into the wire flag; text sessions bypass the hear-the-source-first denoise gate (the source is silence) and snap denoise to 1.0.

Testing

• tests/unit (149 passed; contract drift guards included), npm run typecheck, npm run build.
• Headless WS session against a live pod: silent initial buffer in → non-silent generated slices out (rms ~0.27), live prompt re-encode applied mid-stream.
• Browser: start session → swap to Text to music (denoise snaps to 1, music fills the silent buffer) → live prompt change via Tags panel → swap back to a fixture. Reconnect path re-derives the t2m config from the store sentinel.

🤖 Generated with Claude Code