identity: subject-wide session management — sessions + logout_all (+8 tests)

api.sx gains sessions(Subject) (enumerate a subject's live sessions) and
logout_all(Subject) ("log out everywhere") — revokes and deregisters every
session the subject holds, auditing a logout per session, leaving other
subjects' sessions untouched. Builds on registry.sessions_for. New
tests/session_mgmt.sx. 193/193.

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: 185 / 185 tests passing**
+**Total: 193 / 193 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -20,6 +20,7 @@
 | ✅ | device | 10 | 10 |
 | ✅ | facade | 9 | 9 |
 | ✅ | delegation | 8 | 8 |
+| ✅ | session-mgmt | 8 | 8 |
 
 
 Generated by `lib/identity/conformance.sh`.