Remove exports and methods nothing uses any more

A reference search over every export and method in src/ found code that
only tests still called, or that was never called at all:

- RemoveSceneObjectCommand: the product only removes through the
  multi-object command; the tests now use that one too.
- NodeRegistry.requireNodeDef: a second function of the same name in
  commands/require.ts raises a CommandError instead of throwing, and is
  the one the product uses. The twin invited the wrong import.
- isTextureAsset, listContainers, AssetTabEditor.show,
  FocusManager.destroy, KeyboardBridge.selectAll and the getters
  ColorField.value, ListView.selected, NumberField.number.

Also: Theme.controlActive now matches the resting control colour, and a
new note asks what editing the graph during play would take.

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

src/core/commands/sceneObjectCommands.ts

@@ -111,12 +111,6 @@ export class RemoveSceneObjectsCommand implements Command {
   }
 }
 
-export class RemoveSceneObjectCommand extends RemoveSceneObjectsCommand {
-  constructor(holderId: string, objectId: string) {
-    super(holderId, [objectId]);
-  }
-}
-
 /**
  * The previous position is read at execution time, so a caller that previewed the move must
  * restore the model before executing.