Complex time-stretch: HQ offline render + real-time STFT play mode

[SynthsBack-lab]

Complex time-stretch — HQ offline render + real-time STFT play mode

Draft / RFC — opening early for feedback on the approach before polishing.

Adds a phase-vocoder ("Complex") time-stretch alongside the existing granular one, in three logical commits:

1. Offline HQ render — a Render HQ Stretch region action (right-click → Play Mode context) that renders a warped region through the signalsmith-stretch WASM engine (MIT), following the region's warp markers as a piecewise-linear schedule, and replaces the region with the baked result.
2. no_std Rust port of the Signalsmith Stretch core (complex FFT, dynamic STFT with a Kaiser PR window, 4-way phase prediction with peak-based frequency map + formant preservation) into crates/engine/src/stretch. Derived from signalsmith-stretch v1.3.2 (MIT, © Signalsmith Audio); credited in the module header. 16 unit tests.
3. Real-time play mode — ComplexStretchSequencer drives the ported stretcher live from a region's warp map per block (output pulse span → source-sample span via ppqn_to_seconds; pitch as a semitone transpose). A new algorithm enum field on AudioTimeStretchBox (0 granular, 1 complex, default granular so existing projects are unaffected) routes a region to a parallel per-region sequencer pool that mirrors the granular pool's pre-warm/recycle lifecycle. Exposed as a Complex Engine toggle in the region Play Mode menu.

How to try it

Set an audio region to Timestretch, then Play Mode → Complex Engine. (Real-time playback uses the WASM engine — on the TS engine a Complex region falls back to granular.)

Known limitations (why this is a draft)

• Latency is not compensated. STFT stretching is inherently ~one block latent (~43 ms at the chosen block size); the engine has no per-region PDC yet, so Complex playback lags granular/native by that much. A delay-compensation pass is the natural follow-up.
• WASM engine only for the real-time path; the TS engine would need its own port.
• The offline render's replacement region carries position/duration/gain/label but not yet loop/fade settings.

Testing

104 engine tests pass (incl. the new stretch + sequencer tests); wasm32-unknown-unknown release build clean; verified end-to-end in the browser (region → Complex Engine plays through the WASM engine, no panics, correct pitch-preserving stretch).

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