Terminal construction previously accepted GhosttyTerminalOptions with
dimensions and one scrollback byte limit. Remove the options struct from
the ABI and make ghostty_terminal_new accept columns and rows directly.
Add byte and line limit options to ghostty_terminal_set and forward them
to the runtime Terminal setters. NULL removes a limit, while zero bytes
disables scrollback. Update type metadata, tests, and all API examples.
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.
