fix(evidence): require decodable PNG screenshots

[glanikali]

Summary

• require screenshot evidence to be a fully decodable PNG instead of trusting file signatures alone
• reject truncated, oversized, zero-dimension, and unsupported image payloads
• share PNG signature and dimension parsing between evidence validation and redaction
• consolidate synthetic PNG fixtures across screenshot-related tests

Why

Signature-only validation allowed malformed or truncated PNG files to pass run verification as screenshot evidence. Because proof artifacts are part of the run-bundle contract, verification should fail closed unless the image can actually be decoded.

The new pre-decode byte and pixel limits also bound decoder input before allocation.

Verification

• corepack pnpm release:check
  • 1,073 tests passed
  • 10 tests skipped
  • typecheck passed
  • build passed
  • public-surface scan passed
  • skill validation passed
  • package dry-run passed
• git diff --check upstream/main...HEAD

Fixes #210