cleanupCompleters method
Removes matching completers for typeEntity in groupEntity and bumps
the registration epoch so any in-flight continuations re-wait instead of
resolving against the now-gone registration.
Implementation
void cleanupCompleters(Entity typeEntity, {required Entity groupEntity}) {
completersK[groupEntity]?.removeWhere((e) => e.typeEntity == typeEntity);
final group = _epochsK[groupEntity] ??= {};
group[typeEntity] = (group[typeEntity] ?? 0) + 1;
}