register method
Registers a parent-child relationship for focus IDs to support bubbling.
If focusable is true, the ID is added to the navigation order.
Implementation
void register(String id, {String? parentId, bool focusable = true}) {
if (id == parentId) return; // Prevent cycles
if (TuiTrace.enabled) {
TuiTrace.log(
'focus.register id=$id parentId=$parentId focusable=$focusable ids=$_focusableIds',
);
}
_parents[id] = parentId;
if (focusable && !_focusableIds.contains(id)) {
_focusableIds.add(id);
}
}