fix: avoid panic when truncating summaries at a multi-byte char boundary

src/annotate/converters/mod.rs

@@ -277,8 +277,15 @@ fn truncate_summary(summary: &str, max_len: usize) -> String {
     if trimmed.len() <= max_len {
         trimmed.to_string()
     } else {
-        // Find the last space before max_len to avoid cutting words
-        let truncate_at = trimmed[..max_len].rfind(' ').unwrap_or(max_len);
+        // max_len is a byte count but may land inside a multi-byte UTF-8
+        // character (e.g. box-drawing '─' is 3 bytes) - back off to the
+        // nearest preceding char boundary before slicing.
+        let mut boundary = max_len;
+        while boundary > 0 && !trimmed.is_char_boundary(boundary) {
+            boundary -= 1;
+        }
+        // Find the last space before the boundary to avoid cutting words
+        let truncate_at = trimmed[..boundary].rfind(' ').unwrap_or(boundary);
         format!("{}...", &trimmed[..truncate_at])
     }
 }
@@ -306,6 +313,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_truncate_summary_multibyte_char_at_boundary() {
+        // '─' (U+2500) is 3 bytes in UTF-8, so a naive byte-offset slice at
+        // max_len can land inside the character and panic. This string is
+        // constructed so byte 100 falls inside a '─' run.
+        let summary =
+            "─── Config ─────────────────────────────────────────────────────────────────";
+        // Must not panic, and the result must be valid UTF-8.
+        let _ = truncate_summary(summary, 100);
+    }
+
     #[test]
     fn test_parsed_documentation_getters() {
         let mut doc = ParsedDocumentation::new();