Play through the first camera that is switched on

The game view took whatever camera came first in the scene, ignoring the
switch, so `enabled = false` on a camera did nothing and a second camera
never got its turn. `World.activeCamera()` now decides: of the cameras
switched on – the whole parent chain counted, as for any object – the
first in hierarchy order, and none on renders nothing at all. It is asked
per frame, so a graph switching cameras hands the view over immediately.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/runtime/World.ts

@@ -3,7 +3,7 @@ import { newId } from '../core/ids';
 import type { Asset } from '../core/model/Asset';
 import { isPlaceableType } from '../core/model/AssetTypes';
 import { bakeInstances, materializeInstance } from '../core/model/instance';
-import { findObject } from '../core/model/hierarchy';
+import { findObject, treeOrder } from '../core/model/hierarchy';
 import { descendantsBelow } from '../core/model/walk';
 import { findAsset, findScene, type Project } from '../core/model/Project';
 import { createSceneObject, type SceneObject } from '../core/model/SceneObject';
@@ -274,8 +274,22 @@ export class World {
     return this.entities.find((e) => e.name === name);
   }
 
-  firstOfType(type: Entity['type']): Entity | undefined {
-    return this.entities.find((e) => e.type === type);
+  /**
+   * The camera the game is seen through: of the cameras that are switched on (itself and every
+   * parent, see `properties/enabled.ts`) the first one in hierarchy order. Undefined while none
+   * is on, and then nothing is rendered – switching one off is how a scene hands the view over.
+   */
+  activeCamera(): Entity | undefined {
+    const on = this.entities.filter((e) => e.type === 'camera' && isHandleEnabled(e));
+    if (on.length < 2) return on[0];
+    // Ranked only when the order can decide something, since this runs per frame.
+    const scene = findScene(this.project, this.sceneId);
+    const order = scene ? treeOrder(scene).map(({ object }) => object.id) : [];
+    const rank = (camera: Entity) => {
+      const index = order.indexOf(camera.objectId);
+      return index < 0 ? order.length : index;
+    };
+    return on.reduce((first, camera) => (rank(camera) < rank(first) ? camera : first));
   }
 
   /** Removes an entity and, recursively, everything parented to it. */