v0.22.0

Added

• oxframe check --against (a directory) compares this check's frames with an
  earlier oxframe check --frames output: each frame that changed gets a
  share of changed pixels and diff/<time>.png, the frame dimmed with the
  changed pixels in red; report.json gets a comparison (changed, the
  same, added, removed). Check whether an edit changed only what it meant to.
• oxframe check reports where things move: report.json gets motion,
  each node that travels with its file:line, the stretches of time it
  moves visibly (moves, its keyframes), where it starts and ends, and the
  length of its path; a child that only rides with its parent is left out.
  out/check/trajectories/<scene>.png draws the paths on the scene's last
  frame, numbered as in motion.
• The storyboard shows both sides of a hard cut between scenes: the last
  frame of a scene and the first of the next.
• Image::decode_png.
• The quickstart example: install, oxframe init and oxframe render in a
  terminal, then the rendered scene plays. It is the animation in the
  README's quickstart (scripts/readme-gif.sh makes the GIF).

Tested

• CPU frames are byte-for-byte the same on Linux (x86_64 and arm64), macOS
  (Apple Silicon) and Windows: CI renders ten examples on each and compares
  the hashes.