Add RenderCellStyle plus StyleInto and ResolvedStyleInto for row-cell rendering. The new API lets callers reuse one destination value while reading resolved foreground, background, has-styling, and common text style flags. Previously renderers had to call Style, FgColor, BgColor, and HasStyling separately. That allocated Go wrapper values and crossed cgo multiple times per styled cell. The new binding-side helper batches the existing libghostty queries into one cgo transition and treats absent foreground or background colors as nil fields instead of errors.