Add individual color data kinds to GhosttyRenderStateData so callers
can query background, foreground, cursor color, cursor-color presence,
and the full 256-color palette through ghostty_render_state_get()
without using the sized-struct colors API.
COLOR_CURSOR returns GHOSTTY_INVALID_VALUE when no explicit cursor
color is set; callers can check COLOR_CURSOR_HAS_VALUE first.