Fix tag name case in default trigger detection, refactor engine tests

Tag names from dom-tag-name are lowercase (not uppercase) in the WASM
kernel — fix FORM/INPUT/SELECT/TEXTAREA comparisons in get-default-trigger.

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

shared/static/wasm/sx/engine.sx

@@ -72,12 +72,12 @@
   (fn
     ((tag-name :as string))
     (cond
-      (= tag-name "FORM")
+      (= tag-name "form")
       (list (dict "event" "submit" "modifiers" (dict)))
       (or
-        (= tag-name "INPUT")
-        (= tag-name "SELECT")
-        (= tag-name "TEXTAREA"))
+        (= tag-name "input")
+        (= tag-name "select")
+        (= tag-name "textarea"))
       (list (dict "event" "change" "modifiers" (dict)))
       :else (list (dict "event" "click" "modifiers" (dict))))))