Pond sprites: a clean export, a changed export and a damaged copy

Start with: examples/svarn-audit-pack/ contains three small fictional folders laid out like a Svarn schema-1 export (16×16 PNG squares, not real artwork): export-v1 (frog_1, frog_2, lily_1), export-v2 (frog_2 recoloured, lily_1 removed, dragonfly_1 added) and export-damaged (a copy of export-v1 with one byte flipped inside lily_1.png).

1. In a terminal, change into the extracted svarn-audit-pack folder.
2. Run: python3 audit.py /path/to/examples/svarn-audit-pack/export-v1
3. Run: python3 audit.py /path/to/examples/svarn-audit-pack/export-v1 --compare /path/to/examples/svarn-audit-pack/export-v2
4. Run: python3 audit.py /path/to/examples/svarn-audit-pack/export-damaged

Expected result: 1) "status": "verified" with "files": 3 and exit code 0. 2) Also verified, plus "comparison": added ["dragonfly_1.png"], changed ["frog_2.png"], removed ["lily_1.png"]. 3) AUDIT FAILED: PNG CRC mismatch, exit code 1.

All examples are fictional or synthetic.
