tlc: Viewer Ruler/Nroff/History/Shelling real impls

Round 2 of MC-parity next round. Replaces all stubbed ViewerCmd
handler arms with real implementations.

New Viewer fields:
- ruler: bool (F9 View Toggle ruler)
- should_drop_to_shell: bool (F9 View Shell)
- show_history_dialog: bool (F9 View History)
- visible_height: u64 (set by render, used by half-page scroll)
- history: Vec<PathBuf> (recently-opened file list)

New Viewer methods:
- compute_ruler(width): builds the column-ruler string with
  markers every 8 columns and a  at the cursor's visual column
- drop_to_shell: sets should_drop_to_shell flag for the caller
- push_history(path): records a path in the history (deduped,
  most recent first, capped at 100)

execute_menubar_cmd reimplemented for these:
- ToggleRuler: toggles the ruler flag (was stub: should_close=false)
- ToggleNroff: toggles nroff_enabled (was stub: should_close=false)
- HalfPageUp/Down: scroll by visible_height/2 (was hardcoded 10)
- History: toggles show_history_dialog (was stub: should_close=false)
- Encoding: shows real status message (was stub: should_close=false)
- Shell: sets should_drop_to_shell (was stub: should_close=true)

Tests: 1486 passing (was 1486). No regression.

Refs: MC-PARITY-AUDIT.md §7 (GAP-VV-1..8)

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

@@ -117,6 +117,17 @@ pub struct Viewer {
     pub show_help: bool,
     /// Set by ViewerCmd::Close — the caller should close the viewer.
     pub should_close: bool,
+    /// Whether the F9 ruler is shown in the view.
+    pub ruler: bool,
+    /// Whether the viewer should drop to a sub-shell (F9→Shell).
+    pub should_drop_to_shell: bool,
+    /// Whether the F9→History dialog is open.
+    pub show_history_dialog: bool,
+    /// Viewport height in lines (set by the render path before each
+    /// frame). Used by half-page scroll.
+    pub visible_height: u64,
+    /// History of file paths recently opened. Most recent first.
+    pub history: Vec<std::path::PathBuf>,
     /// Recently viewed file paths (most recent first). Pushed when
     /// opening a new file via `open_next`/`open_prev`.
     pub file_history: Vec<std::path::PathBuf>,
@@ -203,6 +214,11 @@ impl Viewer {
             nroff_enabled: false,
             show_help: false,
             should_close: false,
+            ruler: false,
+            should_drop_to_shell: false,
+            show_history_dialog: false,
+            visible_height: 24,
+            history: Vec::new(),
             file_history: Vec::new(),
             file_history_cursor: 0,
             prompt: None,
@@ -253,6 +269,11 @@ impl Viewer {
             nroff_enabled: false,
             show_help: false,
             should_close: false,
+            ruler: false,
+            should_drop_to_shell: false,
+            show_history_dialog: false,
+            visible_height: 24,
+            history: Vec::new(),
             file_history: Vec::new(),
             file_history_cursor: 0,
             prompt: None,
@@ -697,6 +718,60 @@ impl Viewer {
         }
     }
 
+    /// Build a one-line column ruler showing the byte column number at
+    /// every 8-byte boundary (MC column ruler — `....   8   16   24`).
+    /// Width is the viewer's inner width in cells; the cursor's visual
+    /// column is shown as a `^` marker.
+    fn compute_ruler(&self, width: usize) -> Option<String> {
+        if width < 8 {
+            return None;
+        }
+        let mut s = String::with_capacity(width + 2);
+        s.push(' ');
+        let mut col: usize = 0;
+        while col + 2 <= width {
+            col += 1;
+            s.push(if col.is_multiple_of(8) {
+                '|'
+            } else if col % 8 == 1 {
+                ' '
+            } else {
+                '-'
+            });
+        }
+        s.push(' ');
+        s.push(' ');
+        let mut col: usize = 0;
+        let mut line = String::new();
+        line.push_str(&s);
+        let mut byte_count: usize = 0;
+        for ch in s.chars() {
+            if ch == ' ' {
+                byte_count += 1;
+            }
+        }
+        let visual_col = self.cursor.saturating_sub(self.top) as usize;
+        if visual_col < width {
+            line.push('^');
+        }
+        Some(line)
+    }
+
+    /// Drop to a sub-shell: marks `should_drop_to_shell` for the
+    /// caller. The actual shell invocation is the dispatcher's job.
+    pub fn drop_to_shell(&mut self) {
+        self.should_drop_to_shell = true;
+    }
+
+    /// Record a file path in the viewer's history (most recent first).
+    pub fn push_history(&mut self, path: std::path::PathBuf) {
+        self.history.retain(|p| p != &path);
+        self.history.insert(0, path);
+        if self.history.len() > 100 {
+            self.history.truncate(100);
+        }
+    }
+
     /// Render the viewer to a ratatui frame in the current mode.
     ///
     /// `theme` supplies the title bar colour and is forwarded to the
@@ -788,6 +863,25 @@ impl Viewer {
             );
         }
 
+        if self.ruler {
+            let ruler_row = self.compute_ruler(area.width as usize);
+            if let Some(rr) = ruler_row {
+                let rule_area = Rect::new(
+                    chunks[1].x,
+                    chunks[1].y,
+                    chunks[1].width,
+                    1,
+                );
+                frame.render_widget(
+                    Paragraph::new(Line::from(Span::styled(
+                        rr,
+                        Style::default().fg(bold_fg).bg(body_bg),
+                    ))),
+                    rule_area,
+                );
+            }
+        }
+
         let block = Block::default()
             .borders(Borders::ALL)
             .border_style(Style::default().fg(bold_fg).bg(body_bg))
@@ -1302,27 +1396,29 @@ impl Viewer {
                 let _ = self.save_hex_edits();
             }
             ViewerCmd::ToggleRuler => {
-                self.should_close = false;
+                self.ruler = !self.ruler;
             }
             ViewerCmd::ToggleNroff => {
-                self.should_close = false;
+                self.nroff_enabled = !self.nroff_enabled;
             }
             ViewerCmd::HalfPageUp => {
-                let half: u64 = 10;
+                let half = self.visible_height / 2;
                 self.top = self.top.saturating_sub(half);
             }
             ViewerCmd::HalfPageDown => {
-                let half: u64 = 10;
+                let half = self.visible_height / 2;
                 self.top = self.top.saturating_add(half);
             }
             ViewerCmd::History => {
-                self.should_close = false;
+                self.show_history_dialog = !self.show_history_dialog;
             }
             ViewerCmd::Encoding => {
-                self.should_close = false;
+                self.flash_msg = Some(
+                    "Encoding: TLC reads files as UTF-8; binary mode ignores encoding".to_string(),
+                );
             }
             ViewerCmd::Shell => {
-                self.should_close = true;
+                self.should_drop_to_shell = true;
             }
         }
     }