## Why
The user-verification API previously returned an unsupported provider on macOS. Enable native verification using biometric-protected credentials.
## What changed
- Implement credential status, creation, reuse, deletion, and challenge signing with P-256 Secure Enclave keys in the Data Protection Keychain.
- Require biometric authentication through the key's access-control policy and use a fresh `LAContext` for each signature.
- Report Touch ID hardware support independently of enrollment and current readiness, and map native failures to structured verification errors.
- Serialize credential operations across processes with cancellable file-lock waits. Invalidate pending authentication on cancellation and wait for the signer to exit before releasing the lock.
## Testing
Add unit tests for lock contention and cancellation, authentication teardown and late-result rejection, native error classification, and validation of Secure Enclave key attributes.
GitOrigin-RevId: 93268962cd824b2befc1a0c54b99dcb77a0f1044
## What changed
- Introduce `codex-user-verification` with a provider interface for credential status, creation, deletion, and challenge signing. Include typed errors, shared cancellation guards, and hashed account-user key namespaces.
- Add P-256 public-key encoding as unpadded base64url SPKI DER, derive credential IDs from its SHA-256 digest, and redact proof fields in debug output.
- Add app-server helpers to validate challenge and display-text bounds and map provider errors to typed RPC errors without exposing provider diagnostics.
The platform implementation reports verification as unsupported. App-server requests still return typed unavailability, with the message updated to mention build or account availability.
## Testing
Add tests for credential encoding and signature verification, invalid curve points, cancellation across guard clones, stable and distinct account namespaces, and invalid challenge or display values. Update the app-server unavailability test for the revised message.
GitOrigin-RevId: fe4a4eb37c68d7fdc547704e76aa257abf3e9c81
