identity: dynamic client registration (RFC 7591, +5 tests)

register_dynamic generates a client_id + secret server-side and registers
the client, returning {ok, ClientId, Secret} — self-service onboarding
distinct from the manual register_client. A dynamic confidential client can
then use client_credentials; a dynamic public client stays
unauthorized_client. New tests/dynreg.sx. 222/222.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/identity-on-sx.md

@@ -19,7 +19,7 @@ through the event log, all authorization questions delegated to `acl-on-sx`.
 
 ## Status (rolling)
 
-`bash lib/identity/conformance.sh` → **217/217** (4 phases + 12 ext) — needs `timeout 580`
+`bash lib/identity/conformance.sh` → **222/222** (4 phases + 13 ext) — needs `timeout 580`
 
 ## Ground rules
 
@@ -86,12 +86,19 @@ lib/identity/api.sx ── (identity/login) (identity/grant?) (identity/revoke)
 - [x] acl-on-sx delegation: identity-gates-before-acl boundary (401 vs 403), stub decider (live Datalog bridge is cross-substrate)
 - [~] OAuth `state`/OIDC `nonce` — low value in this server-centric model (client-side echo); skipped
 - [x] pushed authorization requests (PAR, RFC 9126): single-use request_uri → consent
+- [x] dynamic client registration (RFC 7591): server-generated client_id + secret
 - [x] unify `api.sx` over membership + audit (one facade, audited login/logout)
 - [x] subject-wide session management: `sessions(Subject)` + `logout_all` (log out everywhere)
 - [x] token exchange (RFC 8693): downscope a token into a new independent token
 - [x] RFC 7662 full introspection metadata (`introspect_full`: sub/client_id/scope/exp/iat/token_type)
 
 ## Progress log
+- 2026-06-07 — dynamic client registration (ext, RFC 7591): `register_dynamic`
+  generates a client_id + secret server-side (make_ref each) and registers the
+  client, returning {ok, ClientId, Secret} — self-service onboarding distinct
+  from the manual register_client. A dynamic confidential client can then use
+  client_credentials; a dynamic public client stays unauthorized_client. New
+  tests/dynreg.sx (5). 217→222.
 - 2026-06-07 — PAR (ext, RFC 9126): `push_authorization_request` lodges the
   authorization params under a single-use `request_uri`; `authorize_pushed`
   redeems it into the normal consent flow. Pushed requests reuse the pending