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>

lib/persist/scoreboard.md

@@ -8,4 +8,5 @@ _Generated by `lib/persist/conformance.sh`_
 | log | 9 | 0 | 9 |
 | kv | 13 | 0 | 13 |
 | project | 9 | 0 | 9 |
-| **Total** | **37** | **0** | **37** |
+| subscribe | 9 | 0 | 9 |
+| **Total** | **46** | **0** | **46** |