persist: atomic batch append — contiguous block + transactional guard + 10 tests

batch.sx: persist/append-batch commits (type at data) specs as one contiguous
block; persist/append-batch-expect checks the stream is still at expected
before writing any event, so the batch is all-or-nothing under a concurrent
writer (conflict is a value, not a partial write). 162/162.

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

lib/persist/scoreboard.json

@@ -14,9 +14,10 @@
     "cas": {"pass": 11, "fail": 0},
     "catalog": {"pass": 10, "fail": 0},
     "query": {"pass": 9, "fail": 0},
+    "batch": {"pass": 10, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 152,
+  "total_pass": 162,
   "total_fail": 0,
-  "total": 152
+  "total": 162
 }