feat: symusic consolidation + pianoroll UI + route 404 fix

[slittycode]

Summary

Consolidates the backend's symbolic-MIDI handling onto symusic, adds the transcription-pianoroll UI surface, and fixes a live 404 bug on the pianoroll route.

Rebased after #117 merged. The pianoroll endpoint commit in this branch was byte-identical to #117, so it auto-dropped during the rebase onto main (no longer included here). What remains is the genuinely-new work below.

Fix: pianoroll route returns 404 for every real run (LIVE on main)

#117 shipped the endpoint in its pre-fix state. The route reads transcriptionDetail as a nested key, but the pitch-note worker stores it unwrapped as the stage result — so GET /api/analysis-runs/{run_id}/transcription/pianoroll 404s on every real run today. This PR reads it unwrapped (with legacy-wrapper tolerance + a notes-list guard) and unwraps the test fixture so the regression is actually caught.

symusic consolidation (backend)

• Migrate sample_synthesis.write_midi to symusic; add a symusic.Synthesizer audio path behind ASA_SAMPLE_SYNTH_BACKEND (auto|symusic|fluidsynth|sine, FluidSynth-first under auto); migrate research MIDI parsers off pretty_midi; drop pretty_midi + mido. Plus a FluidSynth-vs-symusic parity probe (research-only; skips cleanly without FluidSynth/a SoundFont).

Transcription pianoroll UI

• Canvas heatmap (TranscriptionPianoroll + TranscriptionPianorollBlock) + typed client, mounted in the Session Musician suite; Playwright smoke coverage.

Invariants

Additive only — Phase 1 measurements stay authoritative; the pianoroll is a derived view citing bpm + timeSignature (PURPOSE.md #2).

Testing (post-rebase)

Backend sample_synthesis + pianoroll-route tests 27 OK; frontend tsc clean + pianoroll client unit tests 8 OK; route fix confirmed present on server.py.

🤖 Generated with Claude Code

[slittycode]

Verdict: COMMENT

Summary

Drops mido + pretty_midi in favor of symusic throughout, adds a symusic.Synthesizer backend behind ASA_SAMPLE_SYNTH_BACKEND, and ships the transcription pianoroll surface (GET /api/analysis-runs/{id}/transcription/pianoroll + canvas heatmap). All new pianoroll logic is tested; test_transcription_pianoroll.py and transcriptionPianorollClient.test.ts ran clean in this review. No phase boundary violations, no Phase 1 contract changes.

Findings

Should fix

_resolve_backend docstring contradicts the code — sample_synthesis.py:122-127

The docstring says:

The env var still wins when set, so an operator can explicitly pin a backend even while passing allow_soundfont_backends=False

The code says:

if override == "sine" or not allow_soundfont_backends:
    return "sine_fallback"

allow_soundfont_backends=False triggers the same branch regardless of what the env var says — it returns sine_fallback even with ASA_SAMPLE_SYNTH_BACKEND=symusic. The test test_allow_soundfont_false_forces_sine confirms this is the intended behavior, so the code and test are correct. The docstring is not.

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_build_score_from_plan claims to be shared with write_midi, but write_midi duplicates it — sample_synthesis.py:237-238

The docstring says:

Shared by the symusic synth and write_midi to keep the plan-to-Score translation in one place; future plan extensions only need updating here.

write_midi doesn't call _build_score_from_plan — it contains the identical 14-line Score-building block inline. The docstring is false, and it's the kind of false that actively causes a bug: someone editing _build_score_from_plan to add a bank-select event or change the TPQ will trust the comment, skip write_midi, and ship diverged MIDI output. Either write_midi should call _build_score_from_plan, or the docstring should be corrected to say the two functions are parallel and both need updating.

Test results

• test_transcription_pianoroll.py: 38/38 pass
• transcriptionPianorollClient.test.ts: 8/8 pass
• BackendResolutionTests / SymusicRenderPathTests / WriteMidiTests: couldn't run (full product venv not available in this review environment; PR claims 958 backend tests pass at tip)

Phase boundary check

Clean. The pianoroll module is a pure visualization layer — reads transcriptionDetail from the pitch-note stage, never touches or re-derives Phase 1 measurement values. The HTTP response cites Phase 1 bpm + timeSignature so chain-of-custody (invariant #2) is preserved on the surface.

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