Upload or reconcile a saved key bundle

Stores the user's identity key, signed prekey, one-time prekey batch, and Kyber prekey batch (post-quantum hybrid). Identical retries preserve consumed prekeys. A different identity or signed prekey requires explicit recovery; never automatically revoke saved keys. A prekey material conflict returns key_prekey_conflict; an owner-bound client may verify allocation bounds and retry with fresh IDs while retaining private records.