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/graph/NodeRegistry.ts

@@ -111,12 +111,6 @@ export function nodeDef(type: string): NodeDef | undefined {
   return defs.get(type);
 }
 
-export function requireNodeDef(type: string): NodeDef {
-  const def = defs.get(type);
-  if (!def) throw new Error(`Unknown node type "${type}"`);
-  return def;
-}
-
 export function allNodeDefs(): NodeDef[] {
   return [...defs.values()];
 }