webapi: Attr.ownerDocument follows the owning element

Fixes the failing subtest of WPT /dom/nodes/Node-mutation-adoptNode.html
("Adopting an element ... owner docs of it's attributes"): after
adopting an element into another document, its attribute nodes'
ownerDocument must report the new document.

Attribute nodes are parent-less, so ownerDocument fell through to the
detached-node fallback (the per-frame owner map / main document), which
never changes on adoption. An attribute node with an owning element now
delegates to that element's ownerDocument; detached Attr nodes keep the
old resolution.

Coverage:
- /dom/nodes/Node-mutation-adoptNode.html 1/2 -> 2/2 (fully green)
- /dom/nodes/attributes-namednodemap-cross-document.window.html 0/2 -> 1/2

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

src/browser/webapi/Node.zig

@@ -650,6 +650,14 @@ pub fn ownerDocument(self: *const Node, frame: *const Frame) ?*Document {
         return null;
     }
 
+    // An attribute node has no parent; its owner follows its element's
+    // (including across adoption into another document).
+    if (self._type == .attribute) {
+        if (self._type.attribute._element) |element| {
+            return element.asNode().ownerDocument(frame);
+        }
+    }
+
     // The root of the tree that a node belongs to is its owner.
     var current = self;
     while (current._parent) |parent| {