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.