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.md

@@ -22,5 +22,6 @@ _Generated by `lib/persist/conformance.sh`_
 | upcast | 9 | 0 | 9 |
 | idempotency | 9 | 0 | 9 |
 | global | 11 | 0 | 11 |
+| example-acl | 10 | 0 | 10 |
 | recovery | 6 | 0 | 6 |
-| **Total** | **191** | **0** | **191** |
+| **Total** | **201** | **0** | **201** |