identity: trust-gated federated identity + cross-instance mapping (Phase 4 complete, +13)

federation.sx — peer-asserted subjects, advisory and trust-gated. An
assertion is accepted only from an explicitly trusted peer (else
{error, untrusted}) and is flagged {peer_asserted, Peer}, never promoted to
local authority; acl decides what a peer-asserted identity may do. Cross-
instance subject mapping namespaces remote subjects by peer
({federated, Peer, Remote}) so two peers' "alice" never collide, with
optional explicit aliasing. Adds an audit-completeness test. New
tests/federation.sx. All four phases done — 124/124.

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: 111 / 111 tests passing**
+**Total: 124 / 124 tests passing**
 
 |  | Suite | Pass | Total |
 |---|---|---|---|
@@ -12,7 +12,8 @@
 | ✅ | sso | 10 | 10 |
 | ✅ | membership | 17 | 17 |
 | ✅ | cache | 9 | 9 |
-| ✅ | audit | 10 | 10 |
+| ✅ | audit | 11 | 11 |
+| ✅ | federation | 12 | 12 |
 
 
 Generated by `lib/identity/conformance.sh`.