refactor(cli): split engine.ts into focused modules

[bwyard]

engine.ts is ~1100 lines covering: partToInstrumentDescriptor, instrument dispatchers (12+ types), seqExtras, and the full createScoreEngine closure. Hard to navigate and extend.

Proposed split:

• `engine/descriptor.ts` — partToInstrumentDescriptor + normalizeNoteTransforms + normalizePatternTransforms
• `engine/dispatchers.ts` — per-instrument audio graph builders (kick, snare, synth, bass303, etc.)
• `engine/patch.ts` — PatchProps type + patch/update handlers
• `engine/index.ts` — createScoreEngine, re-exports

No behavior changes. All existing imports stay the same (re-export from index).

Relates to t322 scaling audit.