oauth.sx now owns a session registry. establish creates a subject session;
silent_authorize (OIDC prompt=none §3.1.2.1) asks "does this subject have a
live session?" — if yes it mints a code skipping consent, bound to client +
redirect_uri + PKCE exactly like a consented code; if no it returns
login_required (a negative state, not a login redirect). One session serves
many clients; end_session closes the fast-path. New tests/sso.sx. 75/75.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>