Add hash table invariant assertions and callee filtering

[neysofu]

Summary

• document the new runtime guards for hash table metadata along with the reachable-callable scan correction in the language semantics docs
• add HashTable.assert_metadata and call it across constructors/mutations to keep len, resize_threshold, and bucket relationships sane
• constrain verification_checker::direct_user_callees to entry-reachable blocks and stop scanning past terminators to avoid dead-arm assumptions (plus cover it with a regression test)

Testing

• Not run (not requested)