fix(missions): keep the replay countdown real-time when the speed changes

Moving the replay-speed slider during the pre-launch hold or countdown
rescaled the time left to liftoff, so 0.25x turned T-8 into T-32 and 4x
into T-2. The speed change now skips a replay that has not lifted off;
mission time already flown is still preserved as before.

src/layers/launches/replay.js

@@ -281,7 +281,8 @@ export function createReplay({ state: layerState, services, parts, source }) {
         ? layerState._replayPausedAtMs
         : Date.now();
     for (const [launchId, startedAt] of layerState._animationStarts) {
-      if (!Number.isFinite(startedAt)) continue;
+      // A countdown runs in real time; rescaling it turned T-8 into T-32 at 0.25x.
+      if (!Number.isFinite(startedAt) || startedAt > now) continue;
       const elapsedMissionMs = (now - startedAt) * previousSpeed;
       layerState._animationStarts.set(
         launchId,

src/layers/launches/replay.test.mjs

@@ -0,0 +1,47 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { createState } from './state.js';
+import { createReplay } from './replay.js';
+import { createPaths } from './paths.js';
+
+const noop = () => {};
+
+function replayRig() {
+  const state = createState({
+    services: {
+      overlays: {
+        setOverlayEntries: noop,
+        setOverlaySourceVisible: noop,
+        clearOverlaySource: noop,
+      },
+    },
+  });
+  const parts = {};
+  parts.paths = createPaths({ state, parts });
+  return { state, replay: createReplay({ state, parts }) };
+}
+
+test('changing replay speed during the countdown keeps its real-time liftoff', (t) => {
+  t.mock.timers.enable({ apis: ['Date'], now: 1_000_000 });
+  const { state, replay } = replayRig();
+  const launch = { launchTime: '2026-07-20T10:00:00Z', timeline: [] };
+  state._animationStarts.set('countdown', 1_008_000); // T-8
+  state._animationStarts.set('flying', 990_000); // 10 s into the mission
+
+  replay.setReplaySpeed(0.25);
+
+  const startedAt = state._animationStarts.get('countdown');
+  assert.equal(startedAt, 1_008_000);
+  const countdown = replay.replayState(
+    launch,
+    startedAt,
+    12,
+    28,
+    5400,
+    state._replaySpeed,
+    Date.now(),
+  );
+  assert.equal(countdown.countdownSeconds, 8);
+  // Mission time already flown is still preserved across the change.
+  assert.equal(state._animationStarts.get('flying'), 960_000);
+});