Expose Placement.rect() from the Zig kitty graphics storage as a new
C API function ghostty_kitty_graphics_placement_rect(). It takes the
terminal, image handle, and a positioned placement iterator, and
writes the bounding grid rectangle into a GhosttySelection out param.
Virtual placements return GHOSTTY_NO_VALUE.
Move all opaque handle typedefs (GhosttyTerminal, GhosttyKittyGraphics,
GhosttyRenderState, GhosttySgrParser, GhosttyFormatter, GhosttyOsc*)
into types.h so they are available everywhere without circular includes
and Doxygen renders them in the correct @ingroup sections.
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