W54: ToggleOverwrite editor F9 command (MC CK_InsertOverwrite)

Add ToggleOverwrite EditorCmd variant, wire it into the Options
menu, and add a dispatch handler that toggles the overwrite
flag and shows [OVR]/[INS] in the status message (mirrors the
existing INS key behavior now accessible from the F9 menu).

Added 1 unit test verifying [OVR]→[INS] cycle.

Tests: 1416 pass (was 1415), zero warnings.

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

@@ -89,6 +89,8 @@ pub enum EditorCmd {
     BlockSave,
     /// Close the current buffer (keep editor open).
     Close,
+    /// Toggle insert/overwrite mode (INS key, MC CK_InsertOverwrite).
+    ToggleOverwrite,
 }
 
 /// Outcome of a menu-bar key press.
@@ -261,6 +263,8 @@ impl EditorMenuBar {
                     MenuItem::item("Word wrap", 'p', EditorCmd::ToggleWordWrap),
                     MenuItem::item("Syntax highlight", 'h', EditorCmd::ToggleSyntax),
                     MenuItem::item("Relative line numbers", 'r', EditorCmd::ToggleRelativeLines),
+                    MenuItem::separator(),
+                    MenuItem::item("Insert/Overwrite", 'i', EditorCmd::ToggleOverwrite),
                 ],
             ),
         ];

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

@@ -1545,6 +1545,14 @@ impl Editor {
                     self.message = Some("No selection".to_string());
                 }
             }
+            EditorCmd::ToggleOverwrite => {
+                self.overwrite = !self.overwrite;
+                self.message = Some(if self.overwrite {
+                    "[OVR]".to_string()
+                } else {
+                    "[INS]".to_string()
+                });
+            }
             EditorCmd::Close => {
                 if self.modified {
                     self.mode = Mode::Prompt(PromptKind::SaveBeforeClose);
@@ -3913,5 +3921,17 @@ mod tests {
         let _ = e.dispatch_editor_cmd(crate::editor::menubar::EditorCmd::InsertFile);
         assert!(e.mode.is_prompt());
     }
+
+    #[test]
+    fn toggle_overwrite_cycles_ovr_ins() {
+        let mut e = make_empty();
+        // Default is Insert mode (not overwrite).
+        // Toggle once → overwrite on.
+        let _ = e.dispatch_editor_cmd(crate::editor::menubar::EditorCmd::ToggleOverwrite);
+        assert_eq!(e.message, Some("[OVR]".to_string()));
+        // Toggle again → overwrite off.
+        let _ = e.dispatch_editor_cmd(crate::editor::menubar::EditorCmd::ToggleOverwrite);
+        assert_eq!(e.message, Some("[INS]".to_string()));
+    }
 }