feat: enable arrangement clip placement playback

[britle20]

Summary

• Adds serializable arrangement tracks and ClipInstance model helpers for beat-snapped placement, moving, deletion, and playback range calculation.
• Converts the SONG arrangement shell from static demo clips to real sidebar drag/drop placement, selected instance deletion, movement, and playhead rendering.
• Expands placed hybrid/audio clips into scheduler events for SONG playback and tracks scheduled sample voices so pause/stop can stop long imported audio sources.
• Updates docs and PLANS.md for Arrangement clip placement and playback #41 and adds focused unit tests.

Changes

• Sidebar clip rows are now drag sources for arrangement placement.
• Arrangement tracks are seeded in app state and placed clip instances are kept as serializable state.
• SONG transport schedules arrangement instances instead of the selected PAT clip.
• Imported audio clips are guarded with a clear runtime-cache missing-source error.

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 occupied, then curl -I http://127.0.0.1:5174/ returned 200

Notes / Limitations

• SONG playback currently reuses the existing lookahead loop scheduler across the visible arrangement range. A true non-looping linear song transport that stops exactly at arrangement end is deferred.
• Audio clip instance resize/crop is still deferred because this feature does not add resize handles or duration-aware SampleLoopEvent playback.
• Imported audio bytes remain session-only until IndexedDB/import persistence is implemented.

Closes #41

[britle20]

Follow-up update:

• Fixed SONG-mode sidebar selection so selecting clips/instruments no longer replaces active arrangement playback with PAT clip events or stops playback just because an audio clip was selected.
• Added arrangement loop start/end controls snapped to bar boundaries, with visible loop region markers in the timeline.
• SONG playback now passes the selected arrangement loop range as scheduler loopStartTick/loopEndTick.
• Added arrangement loop range model helpers/tests and updated docs.

Verification rerun:

• 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 occupied, curl -I returned 200

[britle20]

Follow-up update:

• Replaced toolbar loop start/end selects with draggable V-shaped loop handles on the arrangement ruler.
• Removed the pale green full-track loop fill.
• Kept only thin loop boundary lines in the track area and a green connector line between the ruler handles.
• Updated UI design docs for the new loop handle interaction.

Verification rerun:

• 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 occupied, curl -I returned 200

[britle20]

Follow-up update:

• Removed the empty arrangement hint text: "Drag clips from the sidebar into a track."
• Moved the loop handle V markers and connector line upward so they no longer cover the bar numbers.

Verification rerun:

• 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 occupied, curl -I returned 200

[britle20]

Follow-up update:

• Removed the Delete selected toolbar button.
• Arrangement clip instances can now be deleted by right-clicking the clip block, matching the piano roll deletion interaction.
• Kept Delete/Backspace keyboard deletion on focused clip blocks for accessibility.
• Updated docs to describe right-click deletion.

Verification rerun:

• 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 occupied, curl -I returned 200