feat: add synth runtime modulation rack

[ChuxiJ]

Summary

• add a track-level modulation rack to SynthEngine so canonical subtractive LFO settings drive live playback
• map the highest-signal targets first: amp, pan, filterCutoff, and pitch
• cover the new modulation mapping with focused SynthEngine tests

Details

• export a pure createSynthModulationSpec(...) helper alongside the existing runtime-spec mapping
• instantiate Tone modulation nodes (Tremolo, AutoPanner, AutoFilter, Vibrato) per track and preview synth when the subtractive LFO is enabled
• route the synth chain as voice -> optional modulation rack -> gain -> destination
• restart retriggerable modulation nodes on note attacks for live play, preview, and slide playback paths
• leave FM and sampler instruments unchanged in this slice

Validation

• npx tsc --noEmit
• npx vitest run --environment node src/engine/__tests__/SynthEngine.test.ts
• npx vitest run --environment node src/utils/__tests__/trackInstrument.test.ts tests/unit/sessionLaunchModes.test.ts tests/unit/sessionLegato.test.ts
• npm run build
• browser smoke on http://127.0.0.1:4174/: opened Piano Roll, confirmed synth editor renders, toggled LFO on, changed target to amp, and verified the update writes through window.__store.getState().project.tracks[*].instrument.settings.lfo

Notes

• This PR is stacked on top of feat: connect synth runtime to canonical voices #1015.
• The repo's default jsdom Vitest run is currently blocked in this local environment by a Node v18.16.0 + jsdom/html-encoding-sniffer/@exodus/bytes ESM compatibility issue in the shared node_modules tree. The new runtime coverage added in this PR was validated under --environment node, and the existing build stays green.