Let a behavior run while the editor is open

A behavior object carries a "Run in editor" switch next to Enabled. When
it is on, the behavior ticks outside of Play and works on the saved model
directly - Unity semantics: what it moves is a real change, autosaved and
not undoable.

Input stays with the editor, so On key never fires while typing. The
editor run never bakes placed containers into the model, since that would
silently drop their instanceEdits.

syncToModel reports whether it wrote anything, and the dirty flag is only
raised on change - otherwise a behavior moving something every frame
would trigger ~60 saves a second.

src/editor/PlaySession.ts

@@ -115,7 +115,7 @@ export class PlaySession {
       return;
     }
     // Mirror what the systems did into the runtime copy so the panels show live values.
-    if (world.syncToModel()) this.state.preview();
+    if (world.syncToModel().structure) this.state.preview();
     this.state.runtimeTick();
     if (this.host.graphEditor.visible) this.host.graphEditor.canvas.redraw();
   }