Keep custom footnote/endnote marks as text inside the note on DOCX save (#2647)

Found while testing crates/docx/src/write/story.rs:512-530 (InlineObject::NoteRef, the `current_note == Some(..)` branch)

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>

crates/docx/src/write/story.rs

@@ -520,7 +520,12 @@ impl Writer<'_> {
                         self.rev_open(w, props);
                         w.open("w:r", &[]);
                         self.rpr(w, &p);
-                        w.empty(mark, &[]);
+                        // A custom mark is plain text; the placeholder would turn it into a number.
+                        if custom.is_empty() {
+                            w.empty(mark, &[]);
+                        } else {
+                            w.leaf("w:t", &[("xml:space", "preserve")], custom);
+                        }
                         w.close("w:r");
                         self.rev_close(w, props);
                     }

crates/docx/tests/roundtrip.rs

@@ -707,6 +707,32 @@ fn note_never_references_itself() {
     assert_eq!(part_text(&r, Some(*id)).trim(), "The note.");
 }
 
+/// A note with a custom mark keeps it in the note's own text: a plain run with the mark, not the
+/// `w:footnoteRef` / `w:endnoteRef` placeholder that Word replaces by the automatic number.
+#[test]
+fn custom_note_mark_stays_in_the_note() {
+    let mut d = Document::new();
+    let f = d.add_part(PartKind::Footnote, Vec::new());
+    let e = d.add_part(PartKind::Endnote, Vec::new());
+    for (id, kind) in [(f, NoteKind::Footnote), (e, NoteKind::Endnote)] {
+        let mut note = Paragraph::with_text(" The note.", CharProps::default());
+        note.insert_object(0, InlineObject::NoteRef { kind, id, custom: "*".into() }, &CharProps::default()).unwrap();
+        d.parts.get_mut(&id).unwrap().blocks = vec![para_block(note)];
+    }
+    let mut p = Paragraph::with_text("Text", CharProps::default());
+    p.insert_object(4, InlineObject::NoteRef { kind: NoteKind::Footnote, id: f, custom: "*".into() }, &CharProps::default()).unwrap();
+    let end = p.len();
+    p.insert_object(end, InlineObject::NoteRef { kind: NoteKind::Endnote, id: e, custom: "*".into() }, &CharProps::default()).unwrap();
+    d.body = vec![para_block(p)];
+
+    let bytes = wordcraft_docx::write(&d).expect("write");
+    for (file, mark) in [("word/footnotes.xml", "<w:footnoteRef/>"), ("word/endnotes.xml", "<w:endnoteRef/>")] {
+        let xml = part_xml(&bytes, file);
+        assert!(!xml.contains(mark), "{file}: the custom mark became an automatic number: {xml}");
+        assert!(xml.contains(r#"<w:t xml:space="preserve">*</w:t>"#), "{file}: {xml}");
+    }
+}
+
 /// A note read from a Word file starts with its own number (a reference to itself, from the
 /// `w:footnoteRef` mark). Saving writes that mark back exactly once, with its character style, and
 /// never as a `w:footnoteReference`; reading the saved file gives the same note again.