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/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 batch upcast recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch upcast idempotency recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -42,6 +42,7 @@ run_suite() {
 (load "lib/persist/query.sx")
 (load "lib/persist/batch.sx")
 (load "lib/persist/upcast.sx")
+(load "lib/persist/idempotency.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)