Add emoji display width support and width-aware string truncation

mk_wcwidth() returns 0 for emoji codepoints (U+1F300-U+1FAFF) because
they fall through as unassigned. Modern terminals render these as
2-cell-wide characters, causing column misalignment in any application
that relies on mk_wcwidth() for layout.

Add explicit width mappings:
- Emoji pictographs (U+1F300-U+1F9FF, U+1FA00-U+1FAFF): width 2
- Arrows, geometric shapes, dingbats, misc symbols: width 1
- Variation selectors (U+FE00-U+FE0F): width 0

Add utf8_truncate_to_width() which truncates a UTF-8 string to fit
within a target number of display columns, ensuring multi-byte
sequences are never split mid-character.

Author: thaafox

src/utf8.cpp

@@ -299,6 +299,31 @@ std::string utf8_substr (
   return result;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Truncate a UTF-8 string to fit within a target display width.
+// Unlike substr which counts characters, this counts display columns.
+const std::string utf8_truncate_to_width (
+  const std::string& input,
+  unsigned int target_width)
+{
+  unsigned int current_width = 0;
+  std::string::size_type i = 0;
+  std::string::size_type last_safe = 0;
+  unsigned int c;
+
+  while ((c = utf8_next_char (input, i)))
+  {
+    int w = mk_wcwidth (c);
+    if (w < 0) w = 0;
+    if (current_width + w > target_width)
+      break;
+    current_width += w;
+    last_safe = i;
+  }
+
+  return input.substr (0, last_safe);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 int mk_wcwidth(wchar_t ucs)
 {
@@ -316,6 +341,23 @@ int mk_wcwidth(wchar_t ucs)
     if (width == widechar_ambiguous)
       return 1;
 
+    // Emoji pictographs (U+1F300+) — width 2 in modern terminals
+    if ((ucs >= 0x1F300 && ucs <= 0x1F9FF) ||  // Misc Symbols, Pictographs, Emoticons, Supplemental
+        (ucs >= 0x1FA00 && ucs <= 0x1FAFF))    // Symbols and Pictographs Extended-A
+      return 2;
+
+    // Common symbols — width 1 in modern terminals
+    if ((ucs >= 0x2190 && ucs <= 0x21FF) ||    // Arrows
+        (ucs >= 0x2300 && ucs <= 0x23FF) ||    // Misc Technical
+        (ucs >= 0x25A0 && ucs <= 0x25FF) ||    // Geometric Shapes
+        (ucs >= 0x2600 && ucs <= 0x26FF) ||    // Misc Symbols
+        (ucs >= 0x2700 && ucs <= 0x27BF))      // Dingbats
+      return 1;
+
+    // Variation selectors — zero width
+    if (ucs >= 0xFE00 && ucs <= 0xFE0F)
+      return 0;
+
     // All other negative values
     return 0;
 }

src/utf8.h

@@ -39,6 +39,8 @@ unsigned int utf8_width (const std::string& str);
 unsigned int utf8_text_width (const std::string&);
 std::string utf8_substr (const std::string&, unsigned int, unsigned int length = 0);
 
+const std::string utf8_truncate_to_width (const std::string&, unsigned int target_width);
+
 int mk_wcwidth (wchar_t);
 
 #endif