fix: cancel timeline clip interactions when the pointer stream is interrupted

[dazzatronus]

What

A timeline clip drag could stick to the cursor. If the browser never delivered pointerup (for example, releasing over a native context menu, switching apps mid-drag, or a touch scroll takeover), the interaction state machine remained in dragging, so every subsequent hover move kept repositioning the clip. Clicking to recover could start a new interaction over the stale one, leaving the clip stranded at position: fixed with pointer-events: none until a re-render.

How

• Only the primary button starts drags, resizes, and selection clears
• The tracks container captures the pointer, so the release is delivered even outside the window
• pointercancel, or any move without the primary button held, cancels the interaction: the clip is restored to its original position, the ghost is removed, and no command is executed
• A stale interaction is cancelled before a new pointerdown starts one, and on dispose()
• Events are filtered by pointer id, so a second touch cannot drive or complete another pointer's drag
• touch-action: none on clips stops the browser from reclaiming touch drags for scrolling (empty track area still scrolls)

Verify

Normal drag/drop/resize behaviour is unchanged: 29 unit tests (8 new), plus drag-and-drop and drag-cancel exercised in-browser on this branch. To reproduce the lost pointerup deterministically, paste this in the console on any page showing the timeline — no manual dragging needed:

await (async () => {
  const sleep = ms => new Promise(r => setTimeout(r, ms));
  const clips = [...document.querySelectorAll(".ss-clip")];
  const clip = clips[1] || clips[0];
  if (!clip) return console.log("no .ss-clip found");
  const rect = clip.getBoundingClientRect();
  const opt = (x, y, buttons) => ({ bubbles: true, pointerId: 1, isPrimary: true, button: 0, buttons, clientX: x, clientY: y });
  const x = rect.left + Math.min(40, rect.width / 2), y = rect.top + rect.height / 2;
  clip.dispatchEvent(new PointerEvent("pointerdown", opt(x, y, 1)));
  for (let i = 1; i <= 6; i++) { document.dispatchEvent(new PointerEvent("pointermove", opt(x + i * 30, y - i * 25, 1))); await sleep(20); }
  // the release is never dispatched — same as a pointerup eaten by a native menu or app switch
  document.dispatchEvent(new PointerEvent("pointermove", opt(x + 400, y - 300, 0)));
  await sleep(30);
  console.log(clips.some(c => c.style.position === "fixed")
    ? "BUG: clip is glued to the cursor — move the mouse to watch it follow; click to force a drop"
    : "FIXED: drag self-cancelled and the clip was restored");
})();

Verified on 2.13.0: prints BUG and the clip visibly chases the real mouse. On this branch: prints FIXED and the clip snaps back to its original position with no edit applied.

[github-actions]

🎉 This PR is included in version 2.13.1 🎉

The release is available on:

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

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