kitty: truecolor graphics output with a layered HUDHEADmaster

Sixel is a 1987 printer format wearing a terminal costume: 256 colors per
image, no alpha, no compositing, one flat picture. --kitty [scale] takes
the other road on kitty/ghostty/WezTerm/Konsole:

  - the framebuffer ships as raw RGB888, so no per-frame palette build and
    no coarser quantization when a CGB game blows past 256 colors
  - the HUD (title, render rate, frameskip, turbo, live button readout) is
    a *separate* alpha-blended image on z-index 1, composited over the game
    by the terminal instead of being burned into the picture; 'h' toggles
    it, --no-hud starts without it
  - consecutive identical images are dropped, so a pause menu or text box
    stops the traffic dead
  - pixels go over at 1x and the terminal scales them into a cell box, so
    --kitty 6 costs the same bytes as --kitty 1 (padded to whole cells
    first, so the zoom stays exact)

Two transfers: shm hands over the name of a POSIX shared-memory object
(~60 bytes/frame) when we can tell the terminal is local, b64 inlines the
bytes for ssh and tmux (which might be attached from another machine).
--kitty-shm / --kitty-b64 force either. tmux gets DCS passthrough wrapping.
--chrome and --palette work here just like in sixel mode.

Nothing about a graphics stream is human-readable, so tools/kitty_decode.py
reassembles a capture back into images - chunked payloads, tmux wrapping
and all - and can assert a frame is pixel-identical to a --shot:

  tools/kitty_decode.py cap.bin -o frames --check shot.ppm
  -> OK: image 1 matches shot.ppm exactly (160x144)

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