feat: add piano roll synth editor shell

[ChuxiJ]

Summary

• keep piano roll and transport playback routed through canonical track.instrument state with legacy fallback helpers where needed
• add a dedicated SynthInstrumentEditor shell for subtractive and FM instruments inside Piano Roll
• let the piano roll instrument selector switch directly between subtractive presets, FM, and Quick Sampler while always writing canonical instrument state
• add regression coverage for the new editor wiring and the sampler -> fm -> subtractive selector flow

Verification

• npx tsc --noEmit
• npx vitest run tests/unit/SynthInstrumentEditor.test.tsx tests/unit/PianoRoll.test.tsx tests/unit/VirtualKeyboard.test.tsx tests/unit/trackPresets.test.tsx src/utils/__tests__/trackInstrument.test.ts tests/unit/pianoRollContextMenu.test.tsx tests/unit/sessionLaunchModes.test.ts tests/unit/sessionLegato.test.ts
• npm run build

Notes

• FM still falls back to the legacy synth voice in the playback engine for now; this PR adds the canonical FM state path and editor shell, not a full FM voice implementation yet.
• Quick Sampler sample metadata still uses the legacy sampler mirrors for playback, but track identity and editor routing now come from track.instrument.
• Browser smoke automation was attempted locally, but the desktop Playwright/Chrome environment was unreliable in this session, so this update is backed by typecheck, unit coverage, and build verification.

[ChuxiJ]

Triage: Hold — Piano roll synth editor shell. Part of synth UX improvements. Defer until core features stable.

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