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.md

@@ -10,4 +10,5 @@ _Generated by `lib/persist/conformance.sh`_
 | project | 9 | 0 | 9 |
 | subscribe | 9 | 0 | 9 |
 | concurrency | 8 | 0 | 8 |
-| **Total** | **54** | **0** | **54** |
+| snapshot | 11 | 0 | 11 |
+| **Total** | **65** | **0** | **65** |