docs: plan transport pause and playhead

[britle20]

Summary

Adds the feature plan for transport pause/resume behavior and editor playhead rendering.

References #16.

Changes

• Added docs/features/07-transport-pause-and-playhead.md.
• Updated PLANS.md so Transport pause/resume and editor playhead #16 follows Basic Piano Roll #2 Basic Piano Roll in Active Issue Order.
• Clarified pause, stop, runtime tick, and UI playhead rules in audio/UI/testing/architecture docs.
• Marked completed Drum Sequencer and Main DAW UI Shell specs as done.

Testing

• git diff --check
• 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/documentation only.
• No production transport or playhead code is implemented in this PR.
• The implementation issue remains open and should be handled after Basic Piano Roll #2 Basic Piano Roll.