Editing a main component now propagates to all instances:
- Instance children are mapped to component children via componentId
- syncInstances() copies properties from component to instances,
skipping any property marked in the overrides record
- New children added to a component appear in existing instances
- Store calls syncIfInsideComponent after updateNode, commitMove,
commitResize — changes propagate automatically
- Demo creates real instances (Button, Badge) in the App Preview
Tests added: instance creation with child mapping, sync propagation,
override preservation, new child addition, detach.
f77ad708b8