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>

plans/persist-on-sx.md

@@ -99,7 +99,7 @@ lib/persist/backend.sx              lib/persist/api.sx
 - [x] concurrency conflict surfaced as a real result, not a crash
 
 ## Phase 3 — Snapshots + replay
-- [ ] `snapshot.sx` — checkpoint a projection; replay = snapshot + tail
+- [x] `snapshot.sx` — checkpoint a projection; replay = snapshot + tail
 - [ ] compaction policy; replay-determinism tests
 
 ## Phase 4 — Durable backends via kernel IO
@@ -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 3a (65/65).** `snapshot.sx` — a snapshot is a projection state
+  `{:value :seq}` stored in the kv facet under `snapshot/<name>`.
+  `persist/checkpoint` replays + saves; `persist/replay` = snapshot + tail.
+  11 tests assert the headline both ways: snapshot+tail == full replay (value
+  and whole state), plus replay determinism.
 - **Phase 2c (54/54) — Phase 2 complete.** `concurrency.sx` — optimistic
   concurrency: `persist/append-expect b stream expected ...` refuses the append
   if the stream advanced past `expected`, returning a conflict VALUE