compare a screenshot against a baseline, and what that costs the dependency-free promise

[Flatts3000]

The problem

A screenshot is currently an illustration a human looks at. To be an assertion, something has to
compare it against a known-good baseline and fail when it differs.

The proposal

gamebridge shot hall --baseline docs/baselines/hall.png --tolerance 0.02

Exit non-zero when the difference exceeds the tolerance, and write a diff image beside the shot so a
failure is inspectable rather than just red.

The dependency tension, stated up front

gamebridge is deliberately stdlib-only and zero-dependency, and that is a documented property in
its pyproject.toml and README. Image comparison wants Pillow.

Three options:

1. An optional extra (pip install gamebridge[image]), keeping the core install clean.
2. Hand-rolled PNG decoding for the narrow case we emit. Minecraft writes 8-bit RGBA PNGs, and
   zlib is stdlib, so this is smaller than it sounds but is still a decoder to maintain.
3. Refuse, and leave image comparison to whatever the calling project already uses.

Option 1 is probably right, but it is a decision about the package's character, not just a feature.

Notes

• Tolerance has to account for real nondeterminism. Even with frozen weather and time, dithering and
  driver differences move pixels, so an exact-match assertion will be flaky on a different machine.
• Depends on the determinism issues to be worth much: comparing shots that legitimately differ every
  run just produces noise.