feat: implement transport pause and playhead

[britle20]

Summary

Closes #16

Implements distinct paused transport behavior and runtime editor playhead display for the M1 clip editor.

Changes

• Adds paused scheduler state and startTick resume support.
• Exposes audio engine pauseLoop() separately from stopLoop().
• Updates the transport button so playing shows pause, pause preserves the current tick, and stop resets to tick 0.
• Renders a vertical piano roll playhead and a 16-step drum playhead indicator.
• Documents the transport API/runtime playhead behavior.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present

Screenshots

Not included. Dev server was started and the app returned HTTP 200.

Notes / Limitations

• Visual playhead timing is driven by requestAnimationFrame and reads audio engine state; it does not drive audio scheduling.
• Web Audio events already submitted inside the short schedule-ahead window may still sound briefly after pause/stop.
• Vite used port 5174 locally because 5173 was already occupied.

Follow-up

• Pitched instrument selection and Iowa Piano sustain #19 Pitched instrument selection and Iowa Piano sustain remains next in Active Issue Order.