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.json

@@ -3,9 +3,10 @@
     "event": {"pass": 6, "fail": 0},
     "log": {"pass": 9, "fail": 0},
     "kv": {"pass": 13, "fail": 0},
-    "project": {"pass": 9, "fail": 0}
+    "project": {"pass": 9, "fail": 0},
+    "subscribe": {"pass": 9, "fail": 0}
   },
-  "total_pass": 37,
+  "total_pass": 46,
   "total_fail": 0,
-  "total": 37
+  "total": 46
 }