fix(sim): kill the reverse-landing frame jump (D3)

Ascending the altitude dial and landing on the higher-altitude clip jumped ~3
seconds. A baked morph `transitions/{h}__{l}.mp4` spans src@0 (frac 0) ->
dst@~LOOP_TAIL_S (frac 1) and is scrubbed bidirectionally, but the steady loop
element always seeked LOOP_TAIL_S (3.0s) on landing. Descending lands on dst at
its tail (matches the seek — the s0026 loop-from-tail fix); ascending lands on
src at the morph's HEAD (src@0), so seeking the tail was a 3s jump.

Make the loop landing frame direction-aware: pure `HEFScrub.loopLandFrame(dir,
loopTailS)` returns 0 when ascending, loopTailS otherwise. `loadLoop` takes a
`landFrame`, stores it as `dataset.loopStart`, and the timeupdate/ended wrap
handlers honor it (the base clips are already seamless crossfade-loops, so a
reverse landing wraps [0,dur] cleanly). The settle path lands explicitly on the
direction's frame so a fast ascent never jumps even if the heading clip wasn't
preloaded in time. App-side only — no 154-morph LFS re-bake.

Verify: node unit (loopLandFrame) + a new reverse-landing e2e asserting the loop
anchors at the head (loopStart "0"), and the existing forward "loops from tail"
e2e still green (12/12 altitude-lock).

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

simulator/e2e/tests/altitude-lock.spec.ts

@@ -164,6 +164,38 @@ test("a landed clip loops from the morph-tail offset (no jump-back to frame 0)",
   expect(v.t).toBeGreaterThanOrEqual(2.5); // looping from ~3s, not 0
 });
 
+test("ascending re-lands anchored at the clip head (no reverse-landing jump)", async ({ page }) => {
+  // D3: a morph spans src@0 (frac 0) -> dst@~3s (frac 1). Descending lands on dst at its
+  // tail (~3s); ASCENDING lands on src at its HEAD (~0s). The steady loop must continue
+  // from 0 there — seeking to the tail (~3s) was the ~3s reverse-landing jump.
+  await boot(page);
+  await enableVideo(page);                                // base loop only loads when video is shown
+  // Descend one altitude (cosmos -> orbit), then ascend back (orbit -> cosmos): the
+  // ascent is the reverse landing under test.
+  const start = (await page.locator("#scale-name").textContent())!;
+  await wheelOnStage(page, 60);                           // descend
+  await page.waitForFunction((s) => document.querySelector("#scale-name")?.textContent !== s, start, { timeout: 20000 });
+  const mid = (await page.locator("#scale-name").textContent())!;
+  expect(mid).toContain("orbit");
+  await wheelOnStage(page, -60);                          // ascend back
+  await page.waitForFunction((s) => document.querySelector("#scale-name")?.textContent !== s, mid, { timeout: 20000 });
+  expect(await page.locator("#scale-name").textContent()).toContain("cosmos");
+  // The loop element is now anchored at the HEAD (loopStart "0"), playing — not jumped to
+  // the ~3s tail the descending case uses.
+  await page.waitForFunction(
+    () => { const v = document.querySelector("#vid-loop") as HTMLVideoElement; return v.dataset.loopTail === "1" && v.dataset.loopStart === "0"; },
+    null,
+    { timeout: 10000 },
+  );
+  const v = await page.evaluate(() => {
+    const el = document.querySelector("#vid-loop") as HTMLVideoElement;
+    return { loopStart: el.dataset.loopStart, loopTail: el.dataset.loopTail, paused: el.paused };
+  });
+  expect(v.loopTail).toBe("1");            // tail-loop machinery armed
+  expect(v.loopStart).toBe("0");           // anchored at the head, not the morph tail
+  expect(v.paused).toBe(false);            // the loop is actually playing
+});
+
 test("zoom in plays the matching morph, lands locked, and stays put while parked", async ({ page }) => {
   await boot(page);
   const start = (await page.locator("#scale-name").textContent())!;