Wait for a composition's videos and copy and paste its items (#417)

A composition lost the first frames of a video item. Its clock ran
while the player was still loading, in the tab and in the game. The
game only waited for videos once, when it started. A composition that
a behavior starts later spawned its items after that wait.

- The view now records which video sprites it left out for lack of a
  first picture, one entry per object.
- The composition tab holds its playhead while the preview misses a
  video, for at most VIDEO_WAIT_MS (3 s) like the game.
- CompositionSystem holds a composition's clock while one of its live
  items misses its video. The rest of the game keeps running.
- The editor play and the published game pass their view to the run
  as RunServices.pictures.
- VIDEO_WAIT_MS moves to core/model/video.ts, because the runtime
  does not import from viewport/.
- Videos loaded ahead for coming items do not hold the clock.

Items can now be copied and pasted inside the composition tab.

- Ctrl+C keeps the timing of the copied items next to the objects.
- Ctrl+V pastes objects and items in one undo step. The earliest item
  lands at the frame under the pointer, on the track there.
- Without the pointer over the tracks it lands at the playhead, on the
  selected track or on the top track of the selected items.
- Several items keep their distances and tracks to each other.
- The same copy pastes into a scene as plain objects. Objects copied
  from a scene do not paste into a composition, they have no timing.

Tests:
- Unit composition-edit, clipboard, composition-play: 3 new for
  pasting, 1 new for the clock waiting in the game
- e2e 69i (play from the end waits for the video), 69j (paste under
  the pointer and at the playhead), 68d (a composition started from
  On start waits for its video)
- each new check is red without its fix
- README: 2036 editor checks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/model/compositionEdit.ts

@@ -124,6 +124,33 @@ export function movedClips(
   return out && apart(out, lengthOf) ? out : null;
 }
 
+/**
+ * Copied items under the ids `ids` gives them, moved so the earliest starts at `at`: on their tracks,
+ * or, given `row`, the topmost on that track and the others as far below as they were, as far as
+ * there are tracks – a song's paste. An item whose track is gone counts as on the first.
+ */
+export function pastedItems(
+  items: readonly CompositionItem[],
+  ids: ReadonlyMap<string, string>,
+  at: number,
+  trackIds: readonly string[],
+  row?: number,
+): CompositionItem[] {
+  if (items.length === 0) return [];
+  const first = Math.min(...items.map((i) => i.start));
+  const index = new Map(trackIds.map((id, i) => [id, i]));
+  const rows = items.map((i) => index.get(i.track) ?? 0);
+  const top = Math.min(...rows);
+  const bottom = Math.max(...rows);
+  const shift = row === undefined ? 0 : Math.max(-top, Math.min(trackIds.length - 1 - bottom, row - top));
+  return items.map((i, n) => ({
+    ...i,
+    id: ids.get(i.id) ?? i.id,
+    start: Math.max(0, Math.round(at)) + (i.start - first),
+    track: trackIds[rows[n]! + shift] ?? i.track,
+  }));
+}
+
 /** The item with its end edge dragged to make it `wanted` frames long, at least one; its length is its own from then on. */
 export function resizedItem(item: CompositionItem, wanted: number): CompositionItem {
   const { fitMedia: _, ...rest } = item;