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>

plans/persist-on-sx.md

@@ -42,7 +42,7 @@ read models (feeds, indices, audit logs) update incrementally.
 
 ## Status (rolling)
 
-`bash lib/persist/conformance.sh` → **191/191** (Phases 1–4 complete + extensions)
+`bash lib/persist/conformance.sh` → **201/201** (Phases 1–4 complete + extensions + a reference migration)
 
 ## Ground rules
 
@@ -188,7 +188,22 @@ over an in-process disk (the mock-IO harness).
 feed/-log, flow store, mod/audit, search index, acl grants, identity sessions all
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
+**Reference migration:** `lib/persist/examples/acl.sx` is a worked, tested
+template — an ACL-grants store rebuilt on persist (grants/revokes as events,
+current set as a projection, O(1) checks via a materialized view, an audit-window
+query). It carries an explicit BEFORE (hand-rolled ephemeral map) → AFTER
+diff in its header and proves the headline win (grants survive restart) on the
+durable backend. Other subsystem loops copy this pattern; it does not touch the
+real `lib/acl`.
+
 ## Progress log
+- **Reference migration: acl grants (201/201).** `lib/persist/examples/acl.sx` —
+  a worked, in-scope 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. 10 tests, incl. grants surviving restart
+  on the durable backend (the capability the BEFORE version lacked). The pattern
+  other subsystem loops copy.
 - **Ext: global commit ordering (191/191).** `global.sx` — `persist/gappend`
   records a pointer in a reserved `$global` index (its seq = global commit
   position); `read-global`/`project-global` resolve pointers to events in commit