fix(dom): match setAttribute by qualified name, not local name only

Node::set_attribute looked up the existing attribute by local name alone,
while get_attribute and the remove_attribute op both match by qualified
name. A parsed namespaced attribute is stored with a separate prefix
(xlink:href -> prefix="xlink", local="href"), so setAttribute("xlink:href")
never matched it: the update was pushed as a second attribute, getAttribute
kept returning the stale original, and the element serialized xlink:href
twice.

Match set_attribute by qualified_name_eq, consistent with the read/remove
paths, so the update lands in place and no duplicate is created.

Fixes #547

crates/obscura-js/src/runtime.rs

@@ -1662,6 +1662,22 @@ mod tests {
         assert_eq!(v, serde_json::json!("#icon|#icon|id,xlink:href"));
     }
 
+    #[test]
+    fn set_attribute_updates_a_parsed_namespaced_attribute_in_place() {
+        // setAttribute matched the stored attribute by local name only, so a
+        // parsed `xlink:href` (prefix=xlink, local=href) was never found by the
+        // qualified name "xlink:href": the update was pushed as a *second*
+        // attribute, getAttribute kept returning the stale original, and the
+        // element serialized `xlink:href` twice.
+        let mut rt = setup_runtime(
+            r##"<html><body><svg><use id="u" xlink:href="#a"></use></svg></body></html>"##,
+        );
+        let v = rt
+            .evaluate("(function(){var u=document.getElementById('u');u.setAttribute('xlink:href','#b');var dup=(u.outerHTML.match(/xlink:href/g)||[]).length;return u.getAttribute('xlink:href')+'|'+u.getAttributeNS('http://www.w3.org/1999/xlink','href')+'|'+u.getAttributeNames().join(',')+'|'+dup;})()")
+            .unwrap();
+        assert_eq!(v, serde_json::json!("#b|#b|id,xlink:href|1"));
+    }
+
     #[test]
     fn set_attribute_ns_validates_namespace_constraints() {
         let mut rt = setup_runtime("<html><body></body></html>");