example: add c-vt-kitty-graphics

Demonstrates the sys interface for Kitty Graphics Protocol PNG
support. The example installs a PNG decode callback via
ghostty_sys_set, creates a terminal with image storage enabled,
and sends an inline 1x1 PNG image through vt_write. Snippet
markers are wired up to the sys.h doxygen docs.

include/ghostty/vt.h

@@ -98,6 +98,11 @@
  * grid refs to inspect cell codepoints, row wrap state, and cell styles.
  */
 
+/** @example c-vt-kitty-graphics/src/main.c
+ * This example demonstrates how to use the system interface to install a
+ * PNG decoder callback and send a Kitty Graphics Protocol image.
+ */
+
 #ifndef GHOSTTY_VT_H
 #define GHOSTTY_VT_H
 

include/ghostty/vt/sys.h

@@ -28,6 +28,14 @@
  * an implementation. Passing NULL as the value clears the implementation
  * and disables the corresponding feature.
  *
+ * ## Example
+ *
+ * ### Defining a PNG decode callback
+ * @snippet c-vt-kitty-graphics/src/main.c kitty-graphics-decode-png
+ *
+ * ### Installing the callback and sending a PNG image
+ * @snippet c-vt-kitty-graphics/src/main.c kitty-graphics-main
+ *
  * @{
  */