Fix README visual export sizing

[Coding-Dev-Tools]

What changed

• rebuilds the three README PNG visuals to match their SVG viewbox dimensions
• removes the black screenshot padding that made the diagrams render as small cards inside oversized canvases
• adds a regression test that checks each PNG canvas matches its paired SVG

Why

The previous screenshot export process cropped at twice the intended canvas dimensions. The unused image area rendered black in GitHub's README, making every new visual look broken.

Validation

• python -m pytest tests/test_benchmark_evidence.py -q
• ruff check tests/test_benchmark_evidence.py
• visual review of all three corrected PNGs