docs: plan drum step subdivisions

[britle20]

Summary

• Adds a feature spec for drum step subdivisions.
• Documents the 1x, 2x, and 3x subdivision model while preserving the primary 1-16 step labels.
• Updates planning, data model, audio engine, UI, and testing docs.

Changes

• Added docs/features/13-drum-step-subdivisions.md.
• Updated PLANS.md Active Issue Order with issue 32 after issue 30.
• Documented drumStepSubdivision as serializable clip state.
• Documented subdivision tick math: 120, 60, and 40 ticks.
• Added UI guidance for primary step labels plus nested substep buttons.
• Added testing guidance for subdivision tick math and event preservation.

Testing

• git diff --check
• checked referenced feature doc paths in PLANS.md
• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Notes / Limitations

• This is planning only; no production sequencer implementation is included.
• Issue 30's feature spec is still pending in PR 31, so PLANS.md notes that dependency without linking to a missing local file.
• References Drum step subdivisions #32.