persist: worked reference migration — acl grants on persist + 10 tests

examples/acl.sx: a tested template migrating an ACL-grants store from a
hand-rolled ephemeral map to persist — grants/revokes as events, current set as
a projection, O(1) checks via a materialized view, audit via read-window.
Header carries the BEFORE->AFTER diff. Proves grants survive restart on the
durable backend (the capability the BEFORE version lacked). The pattern other
subsystem loops copy; does not touch the real lib/acl. 201/201.

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

lib/persist/scoreboard.json

@@ -18,9 +18,10 @@
     "upcast": {"pass": 9, "fail": 0},
     "idempotency": {"pass": 9, "fail": 0},
     "global": {"pass": 11, "fail": 0},
+    "example-acl": {"pass": 10, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 191,
+  "total_pass": 201,
   "total_fail": 0,
-  "total": 191
+  "total": 201
 }