docs: Add Audio Lifecycle Refactor plan (Task #56)

Created comprehensive design document for refactoring the tracker-synth
relationship to eliminate initialization order dependencies.

Proposed solution: AudioEngine class that manages both synth and tracker
as private members, providing order-independent initialization and clear
ownership semantics.

Three options analyzed:
- Option A: Unified AudioEngine (recommended)
- Option B: Registration Handle System
- Option C: Reference-Counted Resources

Estimated effort: 2-3 weeks with incremental migration path.
Binary size impact: ~500 bytes (acceptable).

See doc/AUDIO_LIFECYCLE_REFACTOR.md for complete design rationale,
implementation plan, and decision matrix.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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