persist: kv compare-and-swap + create-only put + 11 tests

kv.sx: persist/kv-cas sets a key only if its current value equals expected,
else returns {:conflict :expected :actual}; persist/kv-put-new is create-only.
The kv analogue of log append-expect — atomic current-state for sessions, acl
grants, stock counts. 133/133.

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 snapshot compaction durable blob view recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"