Summary

• Adds serializable arrangement length state with a 16-bar default and 1 to 128 bar normalization.
• Adds arrangement toolbar +/- controls and derives ruler, grid width, scroll width, and loop handle bounds from the current length.
• Confirms before shortening when clip placements would extend past the new arrangement end, then removes only those placements.
• Persists arrangement length, migrates older saved documents to 16 bars, and updates related docs/tests.

Testing

• npm run typecheck --if-present
• npm run lint --if-present
• npm run test --if-present
• npm run build --if-present
• Dev server smoke check: npm run dev -- --host 127.0.0.1 --port 5173; Vite used http://127.0.0.1:5174/, and curl -I returned HTTP 200.

Notes / Limitations

• Shortening removes out-of-range arrangement placements after confirmation; it does not trim source clips or clip contents.
• Export duration will use this state in the separate arrangement WAV export task.

Closes #49