Add infinite-scroll swap integration test

Three tests covering the beforeend pagination pattern: page 1 appends
items with sentinel trigger, page 2 appends more, last page shows
"All items loaded" without sentinel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/tests/test-swap-integration.sx

@@ -189,6 +189,48 @@
           (assert-true (contains? result "first"))
           (assert-true (contains? result "second")))))))
 
+(defsuite
+  "swap:infinite-scroll"
+  (deftest
+    "page 1 appends items to container"
+    (reset-mocks!)
+    (set! _mock-args {:page "1"})
+    (let
+      ((page "(div :id \"scroll-items\" (div \"Item 0 — seed\"))")
+        (response (run-handler handler:ex-scroll)))
+      (let
+        ((result (sx-swap page "beforeend" "scroll-items" response)))
+        (do
+          (assert-true (contains? result "Item 0"))
+          (assert-true (contains? result "Item 1"))
+          (assert-true (contains? result "page 1"))
+          (assert-true (contains? result "scroll-sentinel"))))))
+  (deftest
+    "page 2 appends more items"
+    (reset-mocks!)
+    (set! _mock-args {:page "2"})
+    (let
+      ((page "(div :id \"scroll-items\" (div \"page 1 items\"))")
+        (response (run-handler handler:ex-scroll)))
+      (let
+        ((result (sx-swap page "beforeend" "scroll-items" response)))
+        (do
+          (assert-true (contains? result "page 1 items"))
+          (assert-true (contains? result "Item 6"))
+          (assert-true (contains? result "page 2"))))))
+  (deftest
+    "last page has no sentinel"
+    (reset-mocks!)
+    (set! _mock-args {:page "6"})
+    (let
+      ((page "(div :id \"scroll-items\")")
+        (response (run-handler handler:ex-scroll)))
+      (let
+        ((result (sx-swap page "beforeend" "scroll-items" response)))
+        (do
+          (assert-true (contains? result "All items loaded"))
+          (assert-false (contains? result "scroll-sentinel")))))))
+
 (defsuite
   "swap:state-across-calls"
   (deftest