persist: exactly-once append under retries + 9 tests

idempotency.sx: persist/append-once appends at most once per (stream,
idempotency key), returning the same event on a repeat. The marker lives in the
kv facet, so idempotency holds across a restart (verified on durable).
persist/seen? check. 180/180.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/persist/scoreboard.json

@@ -16,9 +16,10 @@
     "query": {"pass": 9, "fail": 0},
     "batch": {"pass": 10, "fail": 0},
     "upcast": {"pass": 9, "fail": 0},
+    "idempotency": {"pass": 9, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 171,
+  "total_pass": 180,
   "total_fail": 0,
-  "total": 171
+  "total": 180
 }