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/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 cas catalog query recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -40,6 +40,7 @@ run_suite() {
 (load "lib/persist/view.sx")
 (load "lib/persist/catalog.sx")
 (load "lib/persist/query.sx")
+(load "lib/persist/batch.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)