fix: confirm deletion for clips used in arrangement

[britle20]

Summary

• Adds a confirmation path when deleting a source clip that has arrangement placements.
• Keeps the existing behavior where imported audio clips and clips with musical events require confirmation.
• Leaves empty, unused hybrid clips deletable without confirmation.

Changes

• Adds a pure clip deletion confirmation helper in the model layer.
• Updates App clip deletion to include arrangement instance count in the confirmation condition.
• Documents the source clip deletion policy in docs/data-model.md.
• Adds unit tests for empty clips, clips with musical events, arranged clips, and imported audio clips with arrangement placements.
• Updates PLANS.md Active Issue Order for Confirm before deleting clips used in arrangement #55.

Testing

• npm run typecheck --if-present: pass
• npm run lint --if-present: pass
• npm run test --if-present: pass, 13 files / 87 tests
• npm run build --if-present: pass

Notes / Limitations

• Manual UI check still recommended: place an empty hybrid clip in arrangement, delete it from the sidebar, and confirm the prompt appears.

Closes #55