persist: snapshots — checkpoint + replay = snapshot + tail + 11 tests

snapshot.sx: snapshot is a projection state {:value :seq} stored in kv under
snapshot/<name>. persist/checkpoint replays and saves; persist/replay folds
only the tail after the snapshot. Tests assert snapshot+tail == full replay
both ways + determinism. 65/65.

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

lib/persist/scoreboard.json

@@ -5,9 +5,10 @@
     "kv": {"pass": 13, "fail": 0},
     "project": {"pass": 9, "fail": 0},
     "subscribe": {"pass": 9, "fail": 0},
-    "concurrency": {"pass": 8, "fail": 0}
+    "concurrency": {"pass": 8, "fail": 0},
+    "snapshot": {"pass": 11, "fail": 0}
   },
-  "total_pass": 54,
+  "total_pass": 65,
   "total_fail": 0,
-  "total": 54
+  "total": 65
 }