refactor: Consolidate instrument engine — unify SynthEngine, SamplerEngine, FM plugin

[ChuxiJ]

Description

Three parallel instrument implementations in different states of maturity:

1. SynthEngine.ts — Legacy 6 hard-coded presets (not parameterizable)
2. SamplerEngine.ts — Sample playback with ADSR (works)
3. fmSynthPlugin.ts — FM synthesis as WAP plugin (works but isolated)

Proposed Architecture

InstrumentEngine (interface)
├── SubtractiveEngine — Maps SubtractiveTrackInstrument to Tone.js
├── SamplerEngine — Refactored to implement interface
├── FMSynthEngine — Extracted from WAP plugin
└── InstrumentFactory — Creates engine from TrackInstrument type

Acceptance Criteria

• Unified InstrumentEngine interface: noteOn, noteOff, setParameter, dispose
• All three types implement same interface
• Legacy SynthEngine.createSynthForPreset() deprecated
• Factory creates correct engine from discriminated union
• Unit tests for each engine type