tlc: phase 16+ — add ButtonKind::Narrow + unicode-safe width tests

After cross-referencing MC's button.c (lib/widget/button.c) and the
ratatui button pattern survey, two gaps remained in Phase 16:

1. MC defines NARROW_BUTTON ([X], no inner padding) — adds for
   future compact toolbar/dialog use. Width formula: label + 2.

2. While width was already chars().count() (Unicode-safe CJK), no
   test asserted this. Added explicit assertion: '日本語' Normal = 7,
   Default = 9 cells.

Tests: 1111 passed (was 1109, +2).
Binaries rebuild clean.

Ref: MC lib/widget/button.c button_flags_t {NORMAL,DEFPUSH,NARROW,HIDDEN}.

local/recipes/tui/tlc/source/src/widget/button.rs

@@ -35,6 +35,9 @@ pub enum ButtonKind {
     /// MC `DEFPUSH_BUTTON` — `[< X >]`. Enter triggers this when
     /// focused.
     Default,
+    /// MC `NARROW_BUTTON` — `[X]` (no inner padding). Used for
+    /// compact dialogs and toolbar items.
+    Narrow,
 }
 
 /// Per-button render spec.
@@ -85,6 +88,7 @@ pub fn render_button(
     let (left, right) = match spec.kind {
         ButtonKind::Normal => ("[ ", " ]"),
         ButtonKind::Default => ("[< ", " >]"),
+        ButtonKind::Narrow => ("[", "]"),
     };
     let mut spans = vec![Span::styled(
         left.to_string(),
@@ -133,14 +137,16 @@ pub fn render_button_row(
     }
 }
 
-/// MC button width formula: `label_chars + 4` (Normal) or
-/// `label_chars + 6` (Default).
+/// MC button width formula: `label_chars + 4` (Normal), `+ 6`
+/// (Default), or `+ 2` (Narrow). All variants use unicode-safe
+/// `chars().count()` so CJK labels compute correctly.
 #[must_use]
 pub fn button_width(spec: ButtonSpec<'_>) -> u16 {
     let label_chars = spec.label.chars().count() as u16;
     let padding = match spec.kind {
         ButtonKind::Normal => 4,
         ButtonKind::Default => 6,
+        ButtonKind::Narrow => 2,
     };
     label_chars + padding
 }
@@ -321,4 +327,37 @@ mod tests {
         assert!(s.starts_with("[ "), "want `[ ...`, got {s:?}");
         assert!(s.trim_end().ends_with(" ]"), "want `... ]`, got {s:?}");
     }
+
+    #[test]
+    fn narrow_button_fused_brackets_no_padding() {
+        // MC NARROW_BUTTON: "[X]" — label + 2, no inner space.
+        let s = render_test(
+            ButtonSpec {
+                label: "OK",
+                hotkey: Some('O'),
+                kind: ButtonKind::Narrow,
+            },
+            false,
+        );
+        assert!(s.starts_with("[OK"), "want `[OK...`, got {s:?}");
+        assert!(s.trim_end().ends_with("K]"), "want `...K]`, got {s:?}");
+    }
+
+    #[test]
+    fn button_width_unicode_safe() {
+        // 3 CJK characters + Normal = 3 + 4 = 7 cells (not bytes).
+        let spec = ButtonSpec {
+            label: "日本語",
+            hotkey: Some('本'),
+            kind: ButtonKind::Normal,
+        };
+        assert_eq!(button_width(spec), 7);
+        // Same with Default variant.
+        let spec_d = ButtonSpec {
+            label: "日本語",
+            hotkey: Some('本'),
+            kind: ButtonKind::Default,
+        };
+        assert_eq!(button_width(spec_d), 9);
+    }
 }