ensureObject<T> method
Returns the object store for T, creating it with one map lookup when
absent.
Implementation
ObjectComponentStore<T> ensureObject<T>() {
final existing = _stores[T];
if (existing != null) return existing as ObjectComponentStore<T>;
final created = ObjectComponentStore<T>();
_stores[T] = created;
_all.add(created);
return created;
}