feat: add synth drive character runtime

[ChuxiJ]

Summary

• add a canonical synth character spec for subtractive filter drive
• insert a dedicated drive/distortion rack into the synth playback chain before modulation and output gain
• cover drive mapping with focused SynthEngine node tests

Validation

• npx tsc --noEmit
• npx vitest run --environment node src/engine/tests/SynthEngine.test.ts
• npm run build
• headless Chromium smoke: app loads, window.__store / window.__uiStore available, Piano Roll opens, synth editor still renders Filter Drive and Filter Envelope

Notes

• this keeps the existing MonoSynth filter gain/exponent mapping and adds an explicit audible character stage on top
• Playwright MCP browser is still blocked on this machine by the existing Chrome persistent-session issue, so browser smoke used inline headless Chromium instead

[ChuxiJ]

Triage: Stacked draft PR — Part of the synth improvement chain (#1015 → #1018 → #1019 → #1060). All are drafts except #1015. Defer until chain base is reviewed.

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