persist: subscription hub — read models update on publish + 9 tests

subscribe.sx: persist/hub wraps a backend; persist/publish appends then fires
per-stream callbacks (backend stream event). Direct persist/append bypasses
subscribers (bulk load/replay). Callbacks drive kv counters / project-resume. 46/46.

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

plans/persist-on-sx.md

@@ -95,7 +95,7 @@ lib/persist/backend.sx              lib/persist/api.sx
 
 ## Phase 2 — Projections + subscriptions
 - [x] `project.sx` — `(project stream step seed)`, incremental fold
-- [ ] subscription hook — projection / kv read model re-runs on append
+- [x] subscription hook — projection / kv read model re-runs on append
 - [ ] concurrency conflict surfaced as a real result, not a crash
 
 ## Phase 3 — Snapshots + replay
@@ -113,6 +113,11 @@ feed/-log, flow store, mod/audit, search index, acl grants, identity sessions al
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Phase 2b (46/46).** `subscribe.sx` — `persist/hub` wraps a backend with
+  per-stream callbacks. `persist/publish` appends then fires subscribers
+  `(backend stream event)`; direct `persist/append` bypasses them by design
+  (bulk load/replay). Canonical use: callback re-runs `project-resume` or bumps
+  a kv counter so read models update on write. 9 tests.
 - **Phase 2a (37/37).** `project.sx` — projection state `{:value :seq}`;
   `persist/project` folds whole stream from seed, `persist/project-resume`
   folds only the tail (seq > prior seq) so read models update incrementally.