example: add grid reference traversal example

Add a c-vt-grid-ref example that demonstrates the terminal and grid
reference APIs end-to-end. The example creates a small 10x3 terminal,
writes text with mixed styles via VT sequences, then iterates over
every cell in the active area using ghostty_terminal_grid_ref. For
each cell it extracts the codepoint, and for each row it inspects
the wrap flag and the style bold attribute.

The grid_ref.h defgroup gains a @snippet reference to the new example,
and vt.h gets the corresponding @example entry and @ref listing.

include/ghostty/vt.h

@@ -50,6 +50,7 @@
  * - @ref c-vt-paste/src/main.c - Paste safety check example
  * - @ref c-vt-sgr/src/main.c - SGR parser example
  * - @ref c-vt-formatter/src/main.c - Terminal formatter example
+ * - @ref c-vt-grid-traverse/src/main.c - Grid traversal example using grid refs
  *
  */
 
@@ -84,6 +85,11 @@
  * contents as plain text.
  */
 
+/** @example c-vt-grid-traverse/src/main.c
+ * This example demonstrates how to traverse the entire terminal grid using
+ * grid refs to inspect cell codepoints, row wrap state, and cell styles.
+ */
+
 #ifndef GHOSTTY_VT_H
 #define GHOSTTY_VT_H
 

include/ghostty/vt/grid_ref.h

@@ -35,6 +35,10 @@ extern "C" {
  * built to sustain the framerates needed for rendering large screens. 
  * Use the render state API for that. 
  *
+ * ## Example
+ *
+ * @snippet c-vt-grid-traverse/src/main.c grid-ref-traverse
+ *
  * @{
  */