feat: add drum step subdivisions

[britle20]

Summary

• Adds clip-level drumStepSubdivision state with supported values 1, 2, and 3.
• Adds model helpers for substep tick math, active-state checks, subdivision updates, and substep toggling.
• Updates the drum sequencer UI with 1x, 2x, and 3x controls while keeping primary step labels 1 through 16.
• Renders nested substep buttons that create DrumEvent.startTick values from primary step and substep positions.
• Updates docs, feature status, Active Issue Order, and model tests.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present
• npm run dev -- --host 127.0.0.1 --port 5173, served on 5174 because 5173 was in use, then curl -I http://127.0.0.1:5174/ returned HTTP 200

Notes / Limitations

• Existing off-grid drum events are preserved when subdivision changes, but only events aligned to the current visible subdivision render as active buttons.
• Wider 2x/3x grids use internal sequencer scrolling if needed.
• Manual listening checks for rhythmic feel were not performed in this environment.

Closes #32