clients.sx (RFC 6749 §2) — confidential clients must present the correct
secret at the token endpoint (wrong → invalid_client); public clients are
identified but not authenticated; redirect_uris are pre-registered and
checked by exact-match valid_redirect (§3.1.2.2 + Security BCP). Standalone
module for now; wiring confidential-client auth into oauth exchange is a
follow-up. New tests/clients.sx. 149/149.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>