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/scoreboard.json

@@ -11,9 +11,10 @@
     "durable": {"pass": 15, "fail": 0},
     "blob": {"pass": 14, "fail": 0},
     "view": {"pass": 11, "fail": 0},
+    "cas": {"pass": 11, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 122,
+  "total_pass": 133,
   "total_fail": 0,
-  "total": 122
+  "total": 133
 }