add Windows fallback fonts for Chinese Kai typeface family

Map MKai PRC, STKai, Kai and related font names to KaiTi,
the standard Kai font bundled with Windows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kjk

ext/mupdf_load_system_font.c

@@ -139,6 +139,17 @@ static struct {
     {"LucidaBright", "Georgia"},
     {"Handwriting", "SegoeScript"},
     {"Console", "Consolas"},
+    // Chinese Kai (regular script) font fallbacks
+    {"MKai PRC", "KaiTi"},
+    {"MKaiPRC-Regular", "KaiTi"},
+    {"MKaiPRC", "KaiTi"},
+    {"STKaiti", "KaiTi"},
+    {"STKaiti-Regular", "KaiTi"},
+    {"STKai", "KaiTi"},
+    {"Kai", "KaiTi"},
+    {"Kaiti_GB2312", "KaiTi"},
+    {"Kaiti SC", "KaiTi"},
+    {"Kaiti TC", "KaiTi"},
 };
 
 static win_fonts g_win_fonts;