Module-level caches keyed by id(obj) return stale results after GC

A memo dict like _cache[(id(world), key)] silently serves results computed for a different object once the original is garbage-collected and CPython reuses its id for a new object (common in loops that build many short-lived worlds, scenarios or models). The bug is invisible in tests that keep objects alive. Attach the cache to the object instead, e.g. obj.__dict__.setdefault('_cache', {}), so it dies with the object; if you must key by id, also store a strong reference to the object in the cache value and check identity (hit[1] is obj) before using it.