acl: Phase 3 explanation + audit, 35 tests

explain.sx reconstructs a canonical proof tree (first-rule, first-solution)
by goal-directed search over the saturated db, since Datalog keeps no
provenance; depth-capped for cyclic safety. acl-explain returns
{:allowed? :proof :reason} with the blocking eff_deny proof on denial.
audit.sx is an append-only decision log (monotonic seq, disk serializer).
api gains acl/explain, acl/audit, acl/audit-tail.

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

lib/acl/scoreboard.json

@@ -1,11 +1,12 @@
 {
   "lang": "acl",
-  "total_passed": 54,
+  "total_passed": 89,
   "total_failed": 0,
-  "total": 54,
+  "total": 89,
   "suites": [
     {"name":"direct","passed":24,"failed":0,"total":24},
-    {"name":"inherit","passed":30,"failed":0,"total":30}
+    {"name":"inherit","passed":30,"failed":0,"total":30},
+    {"name":"explain","passed":35,"failed":0,"total":35}
   ],
-  "generated": "2026-06-06T16:35:53+00:00"
+  "generated": "2026-06-06T16:46:37+00:00"
 }