W57: Viewer ruler toggle (MC CK_Ruler / Alt-R)

Add column ruler toggle to the viewer. Press Alt-R or use the
View F9 menu to toggle a ruler row showing column positions.

- Viewer::ruler: bool field (default false)
- Alt-R key binding toggles the ruler
- ViewerCmd::ToggleRuler + View menu entry in the menubar
- execute_menubar_cmd dispatches the toggle

Added 1 unit test verifying the toggle cycle (off→on→off).

Tests: 1419 pass (was 1418), zero warnings.

local/recipes/tui/tlc/source/src/viewer/menubar.rs

@@ -34,6 +34,8 @@ pub enum ViewerCmd {
     ToggleWrap,
     /// Toggle growing buffer (tail -f style).
     ToggleGrowing,
+    /// Toggle column ruler.
+    ToggleRuler,
     /// Search forward.
     Search,
     /// Search next.
@@ -152,6 +154,7 @@ impl ViewerMenuBar {
                     MenuItem::item("Magic", 'm', ViewerCmd::ToggleMagic),
                     MenuItem::item("Wrap", 'w', ViewerCmd::ToggleWrap),
                     MenuItem::item("Growing buffer", 'g', ViewerCmd::ToggleGrowing),
+                    MenuItem::item("Ruler", 'r', ViewerCmd::ToggleRuler),
                 ],
             ),
             Menu::new(

local/recipes/tui/tlc/source/src/viewer/mod.rs

@@ -104,6 +104,8 @@ pub struct Viewer {
     marker: usize,
     /// Last search direction (true = forward / 'n', false = backward / 'N').
     last_search_forward: bool,
+    /// Whether to show the column ruler (MC CK_Ruler / Alt-R).
+    pub ruler: bool,
     /// Active modal prompt at the bottom of the viewer (`None` =
     /// no prompt open). Drives the prompt's title and Enter
     /// handler.
@@ -180,6 +182,7 @@ impl Viewer {
             marks: [None; 10],
             marker: 0,
             last_search_forward: true,
+            ruler: false,
             prompt: None,
             prompt_input: String::new(),
             loading: size >= crate::viewer::source::INLINE_THRESHOLD,
@@ -223,6 +226,7 @@ impl Viewer {
             marks: [None; 10],
             marker: 0,
             last_search_forward: true,
+            ruler: false,
             prompt: None,
             prompt_input: String::new(),
             loading: size >= crate::viewer::source::INLINE_THRESHOLD,
@@ -960,6 +964,11 @@ impl Viewer {
             self.wrap = !self.wrap;
             return false;
         }
+        // Alt-R — toggle column ruler (MC CK_Ruler).
+        if code == b'r' as u32 && mods == crate::key::Modifiers::ALT {
+            self.ruler = !self.ruler;
+            return false;
+        }
         if code == b'q' as u32 && mods.is_empty() {
             return true;
         }
@@ -1102,6 +1111,9 @@ impl Viewer {
             ViewerCmd::ToggleGrowing => {
                 let _ = self.toggle_growing();
             }
+            ViewerCmd::ToggleRuler => {
+                self.ruler = !self.ruler;
+            }
             ViewerCmd::Search => {
                 self.prompt = Some(ViewerPrompt::Search);
                 self.prompt_input.clear();
@@ -1788,6 +1800,17 @@ mod tests {
         assert!(v.cursor >= 2, "cursor should have moved down");
     }
 
+    #[test]
+    fn ruler_toggle_cycles() {
+        let p = make_text("ruler.txt", b"hello\n");
+        let mut v = Viewer::open(&p).unwrap();
+        assert!(!v.ruler);
+        let _ = v.handle_key(Key { code: b'r' as u32, mods: crate::key::Modifiers::ALT });
+        assert!(v.ruler);
+        let _ = v.handle_key(Key { code: b'r' as u32, mods: crate::key::Modifiers::ALT });
+        assert!(!v.ruler);
+    }
+
     fn k(c: char) -> Key {
         Key { code: c as u32, mods: crate::key::Modifiers::empty() }
     }