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.
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