feat: add timeline track and clip creation APIs

[Errordog2]

Closes #10

Summary

• Add timeline store APIs for creating tracks and clips programmatically.
• Materialize missing intermediate tracks when a requested track id is beyond the current track list.
• Reuse visible same-category tracks for new clips when the target time range has room.
• Mark existing tracks as occupied when a clip is assigned to them and update empty-track labels to the clip category.
• Add a compact left-bar + control for creating a new empty track directly in the timeline UI.
• Add a left-track dropdown for setting editable track categories.
• Add a timeline UI entry point: double-clicking the empty timeline plane creates a clip at the cursor time and on the clicked track/category.
• Add same-category clip movement support with collision rejection for timeline drag interactions.
• Preserve the grab offset during pointer drag so clips move from the point the user picked up instead of jumping to the pointer center.
• Capture and release the pointer during clip drags so the drag can continue while the cursor crosses clip-edge hit areas.
• Add Bun tests for creating empty tracks, creating clips, setting track categories, moving clips, pointer-drag movement, clamping negative movement, rejecting overlap, and rejecting cross-category/category-change moves.
• Fix the local @aitube/clap workspace entrypoint/type exports so Bun can resolve the monorepo package during tests.

Validation

• npx --yes bun --no-install test ./packages/timeline/test/test.tsx passed on head 93bde53: 11 tests, 42 assertions.
• git diff --check main...HEAD passed.

Scope

This adds the store/API foundation for timeline track and clip creation, a direct add-track control, a track category dropdown, double-click clip creation, same-category movement support, pointer-drag behavior that preserves the grabbed offset, and guard coverage for invalid category changes, overlap, and negative movement. Richer clip authoring controls can build on these APIs separately.

[Errordog2]

Validation update after strengthening the edge-case coverage for this bounty:

• New head: 4f5e1f6000f3481a41a616dd7ce6d02ffba7d1ba
• Added regression coverage for three acceptance-sensitive guards:
  • occupied tracks cannot be changed to an incompatible category
  • leftward clip movement clamps at zero while preserving duration
  • same-category moves into overlapping slots are rejected
• git diff --check passed
• npx --yes bun --no-install test ./packages/timeline/test/test.tsx passed: 10 tests, 37 assertions

This keeps the implementation scoped to timeline track/clip authoring and makes the existing compatibility/overlap/clamp behavior easier to verify during review.

[Errordog2]

Follow-up validation for the timeline drag requirements in #10:

• Added pointer-drag handling that preserves the grabbed offset, so clips no longer jump to the pointer center when dragged.
• Removed the requirement that the pointer remain in the clip middle hit area during an active drag, and capture/release the pointer for steadier drag behavior.
• Added regression coverage for dragging a clip from one track to another same-category track while preserving the grabbed offset.

Validation: git diff --check passed with only CRLF normalization warnings; npx --yes bun --no-install test ./packages/timeline/test/test.tsx passed with 11 tests and 42 assertions on remote head 00d58b3.

[Errordog2]

Maintenance update for current head 93bde53:

• Replaced the previous drag-offset fix commit with a clean patch that avoids the large line-ending churn in the prior remote head.
• Re-ran focused validation: npx --yes bun --no-install test ./packages/timeline/test/test.tsx -> 11 tests passed, 42 assertions.
• git diff --check main...HEAD -> passed.

No scope change from the submitted #10 implementation; this keeps the PR easier to review and merge.