fix(reduce-motion): re-roll the clip on an instant-cut altitude change

With reduce motion on, selecting a new altitude jumps (autoScrub's reduced
branch → setPos) and skips the segment that would re-roll the destination clip,
so activeClipId stayed on the previous altitude's clip and the video didn't
change. Self-heal at the landing point: if the landed altitude's pool doesn't
contain activeClipId, pickPoolClip a fresh one. TDD: new altitude-lock e2e.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/static/app.js

@@ -1015,6 +1015,12 @@ function setPos(next) {
   const { lo, frac } = HEFScrub.segmentOf(pos);
   if (frac === 0) {            // settled exactly on an altitude → lock + base loop + single soundtrack
     busy = false;
+    // If we JUMPED straight here (a reduced-motion instant cut skips the segment that
+    // would re-roll the destination), activeClipId still points at the previous
+    // altitude's clip. Re-roll from this altitude's pool so the video actually changes.
+    if (!poolIds(ring.scales[ringIndex]).includes(activeClipId)) {
+      activeClipId = pickPoolClip(ringIndex, activeClipId);
+    }
     delete vid.dataset.morph;  // clear so re-entering the same segment reloads the morph (not the base)
     currentClipId = null;      // force ensureClipMedia to reload + loop the locked clip
     // Land the loop on the frame the morph last showed for THIS travel direction: