chore(justfile): add sixel test recipes

After rebasing onto master (which now carries the justfile from #8),
add recipes for the new configurable encoder + timeout work:

- `sixel-encoder' prints what `kitty-gfx--sixel-resolve-encoder' picks.
- `sixel-encode [ENCODER]' encodes tests/test-image.png headlessly and
  reports payload size; pass an encoder name to force a specific binary
  (e.g. `just sixel-encode img2sixel').
- `sixel-timeout-test' verifies the watchdog kills a `sleep 60' fake
  encoder within `kitty-gfx-sixel-encoder-timeout' seconds.
- `test-sixel-image [ENCODER]' opens tests/test-image.png in Emacs with
  the Sixel backend forced, for manual rendering checks in foot/Konsole/
  mintty/mlterm/WezTerm.

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