Declare the a: and r: namespaces on the modern comments part (#974)

p188:cmLst declared only xmlns:p188 but its comment bodies use a:bodyPr, a:p, a:r and
a:t, so the part was not namespace-well-formed and PowerPoint refused the saved file.
A regression test parses every written XML part with a namespace-aware reader.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/pptx/src/write/mod.rs

@@ -876,7 +876,7 @@ fn author_slot(c: &Comment, authors: &mut Vec<(String, String, u32)>) -> usize {
 /// the flattened legacy part cannot express (issue #70).
 fn modern_comments_xml(list: &[Comment], part: &str, authors: &mut Vec<(String, String, u32)>) -> Vec<u8> {
     let mut w = W::new();
-    w.open("p188:cmLst", A::new().a("xmlns:p188", NS_P188));
+    w.open("p188:cmLst", A::new().a("xmlns:p188", NS_P188).a("xmlns:a", NS_A).a("xmlns:r", NS_R));
     for (ti, c) in list.iter().take(10_000).enumerate() {
         modern_comment(&mut w, c, part, ti, authors, 0);
     }

crates/pptx/tests/comments.rs

@@ -206,3 +206,37 @@ fn modern_only_package_imports() {
     assert!(b[0].resolved);
     assert_eq!((b[0].x, b[0].y), (90.0, 45.0), "modern positions are EMU");
 }
+
+/// Every prefix an exported part uses is bound: PowerPoint refuses a package whose part is not
+/// namespace-well-formed (issue #974, the modern comments part used `a:` without declaring it).
+#[test]
+fn every_written_xml_part_binds_its_prefixes() {
+    use quick_xml::events::Event;
+    use quick_xml::name::ResolveResult;
+
+    let bytes = deckcraft_pptx::export(&deck()).expect("export");
+    let mut z = zip::ZipArchive::new(std::io::Cursor::new(&bytes)).expect("zip");
+    let mut modern = 0;
+    for i in 0..z.len() {
+        let mut f = z.by_index(i).expect("entry");
+        let name = f.name().to_string();
+        if !(name.ends_with(".xml") || name.ends_with(".rels")) {
+            continue;
+        }
+        let mut data = vec![];
+        f.read_to_end(&mut data).expect("read");
+        modern += usize::from(name.contains("modernComment"));
+        let mut r = quick_xml::NsReader::from_reader(data.as_slice());
+        loop {
+            match r.read_resolved_event() {
+                Ok((ns, Event::Start(e) | Event::Empty(e))) => {
+                    assert!(!matches!(ns, ResolveResult::Unknown(_)), "{name}: unbound prefix on <{}>", String::from_utf8_lossy(e.name().as_ref()));
+                }
+                Ok((_, Event::Eof)) => break,
+                Ok(_) => {}
+                Err(e) => panic!("{name}: not well-formed: {e}"),
+            }
+        }
+    }
+    assert!(modern > 0, "the deck should write a modern comments part");
+}