Lowercase word-final Σ as ς in title and sentence case

Found while testing crates/model/src/edit.rs:470-497 (change_case_parts, title/sentence branch)

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

crates/model/src/edit.rs

@@ -469,6 +469,9 @@ pub fn change_case(s: &str, mode: &str) -> String {
 /// piece is the changed form of the matching input piece, even where a character changes length.
 pub fn change_case_parts(parts: &[&str], mode: &str) -> Vec<String> {
     let mut start = true;
+    // All characters of all pieces, so word-final sigma sees neighbours across piece boundaries.
+    let all: Vec<char> = parts.iter().flat_map(|s| s.chars()).collect();
+    let mut base = 0;
     parts
         .iter()
         .map(|s| match mode {
@@ -479,10 +482,14 @@ pub fn change_case_parts(parts: &[&str], mode: &str) -> Vec<String> {
                 // `title` restarts after whitespace, sentence case after sentence punctuation.
                 let title = mode == "title";
                 let mut out = String::new();
-                for c in s.chars() {
+                for (j, c) in s.chars().enumerate() {
+                    let i = base + j;
                     if start && c.is_alphabetic() {
                         out.extend(c.to_uppercase());
                         start = false;
+                    } else if c == 'Σ' && i > 0 && all[i - 1].is_alphabetic() && !all.get(i + 1).is_some_and(|n| n.is_alphabetic()) {
+                        // Word-final capital sigma lowercases to ς, as `str::to_lowercase` does.
+                        out.push('ς');
                     } else {
                         out.extend(c.to_lowercase());
                     }
@@ -490,6 +497,7 @@ pub fn change_case_parts(parts: &[&str], mode: &str) -> Vec<String> {
                         start = true;
                     }
                 }
+                base += s.chars().count();
                 out
             }
         })
@@ -504,6 +512,20 @@ mod tests {
         TextBody::from_text(t)
     }
 
+    #[test]
+    fn title_and_sentence_final_sigma() {
+        assert_eq!(change_case("ΟΔΟΣ", "title"), "Οδος");
+        assert_eq!(change_case("ΟΔΟΣ ΜΟΥ.", "sentence"), "Οδος μου.");
+        assert_eq!(change_case("ΟΔΟΣ", "lower"), "οδος");
+    }
+
+    #[test]
+    fn title_sigma_across_runs_is_not_final() {
+        assert_eq!(change_case_parts(&["ΟΔΟΣ", "ΑΣ"], "title"), ["Οδοσ", "ας"]);
+        assert_eq!(change_case_parts(&["ΟΔΟ", "ΣΑ"], "title"), ["Οδο", "σα"]);
+        assert_eq!(change_case_parts(&["ΟΔΟΣ", " ΜΟΥ"], "title"), ["Οδος", " Μου"]);
+    }
+
     #[test]
     fn insert_and_split() {
         let mut b = body("Hello");