Submits the device key, the platform attestation and a proof of possession. Core verifies the attestation (Apple App Attest on iOS, Android Key Attestation on Android) against the application's deviceAttestation configuration, checks the proof, imports the key as a passkey and returns the credential plus the refreshed MFA state. Attestation is mandatory. Errors - 999995 challenge expired or mismatched, 100011 proof invalid, 100012 attestation failed (the message names the failing step), 100010 platform not configured, 100004 passkey limit reached (checked again under the subject's registration lock, so two concurrent completions cannot exceed the cap), 999998 credentialId already in use (an earlier import or a FusionAuth credential carries it).