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>

lib/identity/scoreboard.md

@@ -1,6 +1,6 @@
 # identity-on-sx Scoreboard
 
-**Total: 217 / 217 tests passing**
+**Total: 222 / 222 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -24,6 +24,7 @@
 | ✅ | exchange | 8 | 8 |
 | ✅ | introspect | 9 | 9 |
 | ✅ | par | 7 | 7 |
+| ✅ | dynreg | 5 | 5 |
 
 
 Generated by `lib/identity/conformance.sh`.