fix(js): keep form-owner lookup root scoped

Resolve controls with a form= attribute inside the current slhx root instead of using document-global id lookup, preserving multi-root isolation for direct handle dispatch.

req: runtime/001

req: convention/004

slhx-js/runtime/slhx.js

@@ -27,10 +27,15 @@
 
   function formOwner(el) {
     if (!el || el.tagName === "FORM") return el;
-    if (el.getAttribute && el.getAttribute("form")) return document.getElementById(el.getAttribute("form"));
+    if (el.getAttribute && el.getAttribute("form")) return elementById(rootOf(el) || document, el.getAttribute("form"));
     return closestInRoot(el, rootOf(el) || document, (node) => node.tagName === "FORM");
   }
 
+  function elementById(scope, id) {
+    if (attrEquals(scope, "id", id)) return scope;
+    return firstElement(scope, (el) => attrEquals(el, "id", id));
+  }
+
   function formHandleId(form) {
     if (!form) return null;
     const holder = form.hasAttribute(HID) ? form : firstElement(form, (el) => el.hasAttribute(HID));

slhx-js/tests/runtime.rs

@@ -94,12 +94,14 @@ fn runtime_handles_get_forms_without_request_body() {
 
 #[test]
 fn runtime_clicking_submitter_schedules_form_submit() {
-    // req: convention/004
+    // req: convention/004 req: runtime/001
     let source = slhx_js::RUNTIME_JS;
 
     assert!(source.contains("function formOwner(el)"));
     assert!(source.contains("function formHandleId(form)"));
-    assert!(source.contains("document.getElementById(el.getAttribute(\"form\"))"));
+    assert!(source.contains("function elementById(scope, id)"));
+    assert!(source.contains("return elementById(rootOf(el) || document, el.getAttribute(\"form\"))"));
+    assert!(!source.contains("document.getElementById(el.getAttribute(\"form\"))"));
     assert!(source.contains("const direct = closestInRoot(event.target, root, (el) => el.hasAttribute(HID))"));
     assert!(source.contains("const submitter = closestInRoot(event.target, root, (el) =>"));
     assert!(source.contains("el.tagName === \"BUTTON\" && (!el.hasAttribute(\"type\") || el.getAttribute(\"type\") === \"submit\")"));