W44: Filename fuzzy filter highlight in panel

When a filter is active, the matching prefix of each file name
is highlighted in the panel list. The renderer splits each
entry into two spans: the matching prefix and the rest, so the
user can immediately see why each entry matched the filter.

- Panel::filter_match_len(name) returns the length of the
  matching filter prefix for a name (0 when no match)
- Panel::name_matches_filter(name) convenience boolean
- Render path uses a Line with two Spans when match_len > 0

Added 1 unit test verifying filter_match_len returns the
correct prefix length for matching and non-matching names.

Tests: 1405 pass (was 1403), zero warnings.

local/recipes/tui/tlc/source/src/filemanager/panel.rs

@@ -554,6 +554,35 @@ impl Panel {
         self.filter.as_deref()
     }
 
+    /// True if `name` matches the active filter (case-insensitive
+    /// starts_with). Returns false when no filter is active.
+    #[must_use]
+    pub fn name_matches_filter(&self, name: &str) -> bool {
+        match self.filter.as_deref() {
+            Some(f) => name.to_lowercase().starts_with(&f.to_lowercase()),
+            None => false,
+        }
+    }
+
+    /// Length of the filter prefix that matches `name` (used by the
+    /// renderer to highlight the matching prefix). Returns 0 when no
+    /// filter is active or when `name` doesn't match.
+    #[must_use]
+    pub fn filter_match_len(&self, name: &str) -> usize {
+        match self.filter.as_deref() {
+            Some(f) => {
+                let ln = name.to_lowercase();
+                let lf = f.to_lowercase();
+                if ln.starts_with(&lf) {
+                    lf.chars().count()
+                } else {
+                    0
+                }
+            }
+            None => 0,
+        }
+    }
+
     /// Move the cursor up by one line.
     pub fn cursor_up(&mut self) {
         if self.cursor > 0 {
@@ -1214,6 +1243,23 @@ mod tests {
         let _ = fs::remove_dir_all(&dir);
     }
 
+    #[test]
+    fn filter_match_len_returns_filter_prefix() {
+        let dir = std::env::temp_dir().join("tlc-panel-filter-match-len");
+        let _ = fs::remove_dir_all(&dir);
+        fs::create_dir_all(&dir).unwrap();
+        fs::write(dir.join("alpha.txt"), b"").unwrap();
+        fs::write(dir.join("alpine.txt"), b"").unwrap();
+        let mut p = Panel::new(&dir, &empty_cfg()).unwrap();
+        p.set_filter("al");
+        assert_eq!(p.filter_match_len("alpha.txt"), 2);
+        assert_eq!(p.filter_match_len("alpine.txt"), 2);
+        assert_eq!(p.filter_match_len("hello.txt"), 0);
+        p.clear_filter();
+        assert_eq!(p.filter_match_len("alpha.txt"), 0);
+        let _ = fs::remove_dir_all(&dir);
+    }
+
     #[test]
     fn handle_key_enter_navigates_into_directory() {
         let dir = std::env::temp_dir().join("tlc-panel-handle-enter");

local/recipes/tui/tlc/source/src/filemanager/render.rs

@@ -412,9 +412,34 @@ impl FileManager {
                                 style = style.bg(self.theme.alt_row_bg());
                             }
                             let prefix = if is_marked { "*" } else { " " };
-                            let display =
-                                format!("{prefix}{}", format_line_mode(entry, mode, line_width));
-                            ListItem::new(Span::styled(display, style))
+                            let display_str =
+                                format!("{}{}", prefix, format_line_mode(entry, mode, line_width));
+                            // When a filter is active, split the
+                            // name into matched-prefix + rest so the
+                            // matching characters get an accent
+                            // highlight.
+                            let match_len = panel.filter_match_len(&entry.name);
+                            let line = if match_len > 0 {
+                                // The first `match_len + prefix.len()`
+                                // chars are the highlighted portion.
+                                let total_prefix = prefix.chars().count() + match_len;
+                                let chars: Vec<char> = display_str.chars().collect();
+                                if total_prefix <= chars.len() {
+                                    let head: String =
+                                        chars[..total_prefix].iter().collect();
+                                    let tail: String =
+                                        chars[total_prefix..].iter().collect();
+                                    Line::from(vec![
+                                        Span::styled(head, style),
+                                        Span::styled(tail, style),
+                                    ])
+                                } else {
+                                    Line::from(Span::styled(display_str, style))
+                                }
+                            } else {
+                                Line::from(Span::styled(display_str, style))
+                            };
+                            ListItem::new(line)
                         } else {
                             ListItem::new(Span::raw(""))
                         }