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/runtime/systems/CompositionSystem.ts

@@ -13,7 +13,7 @@ import { curveLookup } from '../../core/model/curve';
 import { contentId, placedWithContent } from '../../core/model/instance';
 import { findAssetOf } from '../../core/model/Project';
 import type { PropertyValue } from '../../core/model/Properties';
-import { findVideo } from '../../core/model/video';
+import { findVideo, VIDEO_WAIT_MS } from '../../core/model/video';
 import { VIDEO_CLOCK_KEY, VIDEO_TIME_KEY, videoDrivenOf } from '../../core/properties/videos';
 import type { Entity } from '../Entity';
 import type { World } from '../World';
@@ -38,6 +38,14 @@ interface Run {
   id: number;
   /** By item id. An item a behavior destroyed stays here until it ends, so it is not built again. */
   live: Map<string, Spawned>;
+  /** Seconds the clock has stood so far for a video without its first picture, see `waits`. */
+  waited: number;
+}
+
+/** What the view that shows a run knows of its driven videos (#404). */
+export interface VideoPictures {
+  /** Whether it left the sprite `objectId` out, its video still without a first picture. */
+  missing(objectId: string): boolean;
 }
 
 /**
@@ -46,12 +54,18 @@ interface Run {
  * videos. Runs before the animations and the behaviors, so a behavior writes last and hears this
  * frame's `finished`. The editor run plays none: it writes into the saved scene, where the objects of
  * a cut would stay behind.
+ *
+ * While the view still waits for the first picture of a video an item brings in, the composition's
+ * clock stands, so the video starts at its first frame and not where the clock has got to meanwhile.
  */
 export class CompositionSystem implements System {
   finished: CompositionFinish[] = [];
   private readonly runs = new Map<Entity, Run>();
 
-  constructor(private readonly plays = true) {}
+  constructor(
+    private readonly plays = true,
+    private readonly pictures: VideoPictures | null = null,
+  ) {}
 
   update(world: World, dt: number): void {
     this.finished = [];
@@ -82,8 +96,8 @@ export class CompositionSystem implements System {
       return;
     }
     run = this.runs.get(placement);
-    if (!run) this.runs.set(placement, (run = { id: player.run, live: new Map() }));
-    if (player.shown && isHandleEnabled(placement)) player.time += dt;
+    if (!run) this.runs.set(placement, (run = { id: player.run, live: new Map(), waited: 0 }));
+    if (player.shown && isHandleEnabled(placement) && !this.waits(run, dt)) player.time += dt;
     player.shown = true;
     const length = compositionLength(world.project, comp);
     let frame = compositionFrameAt(comp, player.time);
@@ -99,6 +113,20 @@ export class CompositionSystem implements System {
     this.show(world, comp, placement, run, frame);
   }
 
+  /** Whether a video of a live item still has no picture; at most `VIDEO_WAIT_MS` at a stretch. */
+  private waits(run: Run, dt: number): boolean {
+    const pictures = this.pictures;
+    const missing =
+      pictures !== null &&
+      [...run.live.values()].some(({ videos }) => videos.some((v) => pictures.missing(v.objectId)));
+    if (!missing) {
+      run.waited = 0;
+      return false;
+    }
+    run.waited += dt;
+    return run.waited * 1000 < VIDEO_WAIT_MS;
+  }
+
   /** Takes every item of the placement's run away. */
   private end(world: World, placement: Entity): void {
     const run = this.runs.get(placement);