webapi: document.head matches by local name; getElementsByName is HTML-only

Fixes two WPT files (1 subtest each, both fully green):

- /html/dom/documents/dom-tree-accessors/document.head-02.html:
  document.head is the first html-namespace child of the document
  element whose LOCAL name is head. A createElementNS(HTML_NS,
  "blah:head") element is an HTMLUnknownElement in our type system, so
  the old is(Html.Head) walk skipped it; getHead now matches on
  namespace + local name (and returns *Element accordingly).
- .../document.getElementsByName/document.getElementsByName-namespace.html:
  getElementsByName must only consider HTML-namespace elements; the
  live collection's name filter now rejects foreign elements, matching
  the earlier fix for name-based access on HTMLCollections.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/browser/webapi/HTMLDocument.zig

@@ -44,14 +44,17 @@ pub fn asEventTarget(self: *HTMLDocument) *@import("EventTarget.zig") {
 }
 
 // HTML-specific accessors
-pub fn getHead(self: *HTMLDocument) ?*Element.Html.Head {
+// The head is the first html-namespace child of the document element whose
+// local name is head, whatever its Zig element type (e.g. a createElementNS
+// "blah:head" is an HTMLUnknownElement but still qualifies).
+pub fn getHead(self: *HTMLDocument) ?*Element {
     const doc_el = self._proto.getDocumentElement() orelse return null;
     var child = doc_el.asNode().firstChild();
-    while (child) |node| {
-        if (node.is(Element.Html.Head)) |head| {
-            return head;
+    while (child) |node| : (child = node.nextSibling()) {
+        const el = node.is(Element) orelse continue;
+        if (el._namespace == .html and std.mem.eql(u8, el.getLocalName(), "head")) {
+            return el;
         }
-        child = node.nextSibling();
     }
     return null;
 }

src/browser/webapi/collections/node_live.zig

@@ -283,6 +283,8 @@ pub fn NodeLive(comptime mode: Mode) type {
                 },
                 .name => {
                     const el = node.is(Element) orelse return false;
+                    // getElementsByName only considers HTML elements.
+                    if (el._namespace != .html) return false;
                     const name_attr = el.getAttributeSafe(comptime .wrap("name")) orelse return false;
                     return std.mem.eql(u8, name_attr, self._filter);
                 },