Nine tests covering INDEX16 (normal + clipmap), P8, A8R8G8B8, R8G8B8,
A8Additive (matches our current DecodeA8), A8 non-additive (documents
the divergence), R5G6B5, A4R4G4B4. All run before any substitution --
they prove equivalence, not test the substitution.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
