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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency)
+SUITES=(event log kv project subscribe concurrency snapshot)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -33,6 +33,7 @@ run_suite() {
 (load "lib/persist/kv.sx")
 (load "lib/persist/project.sx")
 (load "lib/persist/concurrency.sx")
+(load "lib/persist/snapshot.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)