Set gravity survives a re-read of the model during play: World.setSceneProp remembers what the run itself set on the scene and syncFromModel lays it back over the copied scene props, so a project event mid-play no longer resets the gravity to the saved 980; the unit test re-reads the model mid-run and e2e 41c forces a project event right after play starts (#147)

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

src/runtime/physics/PhysicsSystem.ts

@@ -216,8 +216,8 @@ export class PhysicsSystem implements System, CollisionSource {
 
   /** The scene's gravity for the rest of this run, in world units per second squared. */
   setGravity(world: World, gravity: Vec2): void {
-    world.sceneProps.gravityX = gravity.x;
-    world.sceneProps.gravityY = gravity.y;
+    world.setSceneProp('gravityX', gravity.x);
+    world.setSceneProp('gravityY', gravity.y);
     if (this.engine) this.engine.gravity = this.gravity(world);
   }