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/tests/idempotency.sx

@@ -0,0 +1,92 @@
+; Extension — exactly-once append under retries.
+
+(persist-test
+  "seen? false before first append"
+  (persist/seen? (persist/open) "orders" "cmd-1")
+  false)
+(persist-test
+  "append-once appends on first use"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/count b "orders")))
+  1)
+(persist-test
+  "seen? true after first append"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/seen? b "orders" "cmd-1")))
+  true)
+(persist-test
+  "repeat with same key does not append again"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/count b "orders")))
+  1)
+(persist-test
+  "repeat returns the same event (same seq)"
+  (let
+    ((b (persist/open)))
+    (let
+      ((e1 (persist/append-once b "orders" "cmd-1" "placed" 0 {})))
+      (persist/event-seq
+        (persist/append-once b "orders" "cmd-1" "placed" 0 {}))))
+  1)
+(persist-test
+  "different keys append separately"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append-once b "orders" "cmd-1" "placed" 0 {})
+      (persist/append-once b "orders" "cmd-2" "placed" 0 {})
+      (persist/count b "orders")))
+  2)
+(persist-test
+  "idempotency is per-stream"
+  (let
+    ((b (persist/open)))
+    (begin
+      (persist/append-once b "a" "cmd-1" "x" 0 {})
+      (persist/append-once b "b" "cmd-1" "x" 0 {})
+      (list (persist/count b "a") (persist/count b "b"))))
+  (list 1 1))
+(persist-test
+  "stored data is preserved on first append"
+  (let
+    ((b (persist/open)))
+    (get
+      (persist/event-data
+        (persist/append-once b "s" "k" "x" 0 {:n 9}))
+      :n))
+  9)
+(persist-test
+  "idempotency survives restart on the durable backend"
+  (let
+    ((disk (persist/mem-backend)))
+    (begin
+      (persist/append-once
+        (persist/mock-durable disk)
+        "orders"
+        "cmd-1"
+        "placed"
+        0
+        {})
+      (let
+        ((db2 (persist/mock-durable disk)))
+        (begin
+          (persist/append-once
+            db2
+            "orders"
+            "cmd-1"
+            "placed"
+            0
+            {})
+          (persist/count db2 "orders")))))
+  1)