feat: expose filter envelope controls in synth editor

[ChuxiJ]

Summary

• add subtractive filter envelope controls to the Piano Roll synth editor so filter ADSR and amount are editable from canonical track.instrument
• reorganize the subtractive motion section into clearer filter, filter-envelope, LFO/glide, and unison groups
• extend the focused editor test to cover filter envelope amount and release writes

Why

This closes the Sprint 01 gap between runtime support and editor affordance: filterEnvelope already drives subtractive playback, but the UI did not expose those parameters. It also advances the research-backed Phase 2 synth editor scope (Filter ADSR + LFO with routing).

Verification

• npx tsc --noEmit
• npm run build
• browser smoke via headless Playwright script:
  • create a project
  • add a keyboard pianoRoll track
  • open Piano Roll
  • edit Filter Env Amt to 0.42
  • edit Filt Env Release to 1.8
  • verify window.__store.getState().project.tracks[*].instrument.settings.filterEnvelope reflects the new values

Known limitation

• npx vitest run tests/unit/SynthInstrumentEditor.test.tsx is still blocked on this machine by the existing Node 18 + jsdom + @exodus/bytes worker startup issue. The test file was updated, but local execution could not complete until that environment issue is resolved.

[ChuxiJ]

Triage: Merge candidate — Filter envelope controls for synth editor. Standalone feature, good UX improvement. Needs rebase check.

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