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/model/Asset.ts

@@ -86,7 +86,3 @@ export type Asset =
 export function isSceneAsset(asset: Asset): asset is SceneAsset {
   return asset.type === 'scene';
 }
-
-export function isTextureAsset(asset: Asset): asset is TextureAsset {
-  return asset.type === 'texture';
-}

src/core/model/Project.ts

@@ -64,7 +64,3 @@ export function findObjectHolder(project: Project, holderId: string): ObjectHold
   const asset = findAsset(project, holderId);
   return asset && isObjectHolder(asset) ? asset : undefined;
 }
-
-export function listContainers(project: Project): ContainerAsset[] {
-  return project.assets.filter((a): a is ContainerAsset => a.type === 'container');
-}