Play switches to scene tab; graph tab strictly follows selection

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/editor/App.ts

@@ -256,6 +256,8 @@ class Editor {
     this.world.addSystem(this.behaviors);
     this.viewport.setWorld(this.world);
     this.state.setMode('play');
+    // Play always starts on the scene; the graph tab stays reachable for live tracing.
+    this.showTab('scene');
     this.state.setNotice(this.world.firstOfType('camera') ? 'Playing · Esc stops' : 'This scene has no camera – nothing is rendered · Esc stops');
     if (this.world.stopRequested) this.stop();
   }