A note's name is one rule, pinned by the shared fixture

The server and the core each derived display_title and disagreed twice: the
server cut it at 200 characters and the core didn't, and the server split lines
with splitlines(), which also breaks on a lone \r or a U+2028, where the core and
every other reading of the grammar split on \n alone.

- grammar.json gains a display_titles section: blank lines, markers, an empty
  item, \r\n, a lone \r, U+2028, and a 201-character line of 'é' (the cut is
  characters, not bytes).
- derive::display_title and DISPLAY_TITLE_CAP are the core's half, moved next to
  strip_marker. The server splits on "\n". Both suites run the cases.

Behaviour: a device now names a note with a first line over 200 characters the
way the web always has, and the server names a note containing a lone \r or a
U+2028 the way devices always have.

Fixes #5398. DRY pass #2, batch 3 (#5372).

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

core/src/local/derive.rs

@@ -341,6 +341,32 @@ fn render_task_line(indent: &str, bullet: char, checked: bool, text: &str) -> St
     }
 }
 
+/// The longest a note's name runs, in characters (not bytes): the server's
+/// `DISPLAY_TITLE_CAP`.
+pub const DISPLAY_TITLE_CAP: usize = 200;
+
+/// The note's NAME: the first line of its body that says anything, cut to
+/// [`DISPLAY_TITLE_CAP`] characters.
+///
+/// Mirrors `derive_display_title` in the server's notes/helpers.py — one rule written
+/// twice, and they have to agree or a synced note is called different things on either
+/// side of the wire. Both run the `display_titles` cases in testdata/grammar.json.
+///
+/// It no longer needs the items, because the items ARE lines of the body now (M304).
+/// What it needs instead is to strip the task marker off: a list-only note is still
+/// named by its first item, and calling that note "- [ ] milk" would be showing
+/// someone the storage rather than the note. An empty item is skipped rather than
+/// naming the note "", which is what a half-typed list would otherwise do.
+pub fn display_title(body: &str) -> String {
+    for line in body.split('\n') {
+        let text = strip_marker(line.trim()).trim();
+        if !text.is_empty() {
+            return text.chars().take(DISPLAY_TITLE_CAP).collect();
+        }
+    }
+    String::new()
+}
+
 /// The text of a line with its task marker removed, or the line as it was.
 ///
 /// For naming a note: a list-only note is named by its first item, and calling one
@@ -759,6 +785,15 @@ mod tests {
         }
     }
 
+    #[test]
+    fn fixture_display_titles() {
+        for case in fixture()["display_titles"].as_array().unwrap() {
+            let body = case["body"].as_str().unwrap();
+            let want = case["title"].as_str().unwrap();
+            assert_eq!(display_title(body), want, "body {body:?}");
+        }
+    }
+
     #[test]
     fn fixture_lifts() {
         for case in fixture()["lifts"].as_array().unwrap() {