fix: defer timeline pointer capture until drag starts so clip child clicks work

[dazzatronus]

What

Clicking the luma mask badge on a timeline clip no longer detached the mask. Capturing the pointer to the tracks container on every clip pointerdown retargets the derived click event away from clip children, so the badge's click handler never fired.

Pointer capture now happens only when a gesture commits: immediately for resize, and at the drag threshold for drags. Plain clicks flow to clip children untouched.

Why it's safe

The interruption handling that introduced the capture is preserved: pointercancel and the lost-pointerup buttons check are unchanged, and drag/resize (the only states with visible stuck-clip risk) still capture. A lost pointerup during the pending state has no visual side effects and is cleared by the existing cancel paths. All six interruption tests pass unchanged.

Verify

• New test: no capture on plain pointerdown, capture once the drag threshold is crossed.
• Manual: attach a luma to a clip, click the ◐ badge → mask detaches to a new track. Drag a clip and release outside the window → no stuck clip.

[github-actions]

🎉 This PR is included in version 2.15.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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