Serialize / deserialize COSE public keys (integer-keyed maps returned by wax_ via Wax.AttestedCredentialData.credential_public_key).

Uses Erlang External Term Format (:erlang.term_to_binary/1) because:

• wax_ returns %{integer() => ...} — Jason cannot encode integer keys.
• CBOR would be spec-correct but adds a dependency for data wax_ already decoded from CBOR upstream.
• ETF preserves integer keys exactly and round-trips through Cloak encrypt/decrypt unchanged.

The :safe flag on binary_to_term/2 is mandatory — it blocks atom-creation DoS on tampered ciphertext (decrypt still succeeds for authenticated ciphertext under Cloak AES-GCM, but defense-in-depth).