fix(sim): loop base from morph-tail offset so landed morphs don't jitter

Operator eyeball feedback: landing on an altitude jump-cut the loop back to frame 0.
Each morph reproduces the destination clip's first 3s (pipeline trim=0:3) and ends on
dst@~3s, so the steady-state loop now starts at LOOP_TAIL_S=3.0 and loops within
[3.0, duration] — the morph's last frame hands straight off to the loop, and the
morph-covered intro never re-shows at rest (removes the overlapping 'altitude video
that is part of the morph'). E2E asserts a landed clip loops from ~3s, not 0.

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

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

@@ -39,6 +39,16 @@ async function enableAudio(page: Page) {
   });
 }
 
+// The Video toggle gates the base-clip loop (update() only loads media when video is
+// shown). Same hidden-checkbox pattern.
+async function enableVideo(page: Page) {
+  await page.evaluate(() => {
+    const c = document.querySelector("#visual") as HTMLInputElement;
+    c.checked = true;
+    c.dispatchEvent(new Event("change", { bubbles: true }));
+  });
+}
+
 test("two audio elements exist for crossfade", async ({ page }) => {
   await boot(page);
   const n = await page.evaluate(() => document.querySelectorAll("audio#aud, audio#aud-b").length);
@@ -77,6 +87,28 @@ test("wheel auto-scrubs one altitude and locks", async ({ page }) => {
   expect(await page.locator("#scale-name").textContent()).toBe(landed);    // locked
 });
 
+test("a landed clip loops from the morph-tail offset (no jump-back to frame 0)", async ({ page }) => {
+  await boot(page);
+  await enableVideo(page);                                // base loop only loads when video is shown
+  const start = (await page.locator("#scale-name").textContent())!;
+  await wheelOnStage(page, 60);
+  await page.waitForFunction((s) => document.querySelector("#scale-name")?.textContent !== s, start, { timeout: 20000 });
+  // After landing, the base loop is armed (loopTail="1") and seeked to the tail
+  // (~3s) so it continues the morph's last frame instead of restarting at 0.
+  await page.waitForFunction(
+    () => { const v = document.querySelector("video") as HTMLVideoElement; return v.dataset.loopTail === "1" && v.currentTime >= 2.5; },
+    null,
+    { timeout: 10000 },
+  );
+  const v = await page.evaluate(() => {
+    const el = document.querySelector("video") as HTMLVideoElement;
+    return { t: el.currentTime, loopTail: el.dataset.loopTail, morph: el.dataset.morph || "" };
+  });
+  expect(v.morph).toBe("");                // on the base loop, not a morph
+  expect(v.loopTail).toBe("1");            // tail-loop armed
+  expect(v.t).toBeGreaterThanOrEqual(2.5); // looping from ~3s, not 0
+});
+
 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())!;