New DOUBLE LORES test converts gradient-80x48.bmp with "DL N" and
diffs against the checked-in GRADIENT.DLORES reference. Add a
.gitignore for the built binary and generated .SLO/.DLO outputs.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BCcEM6zYoVcXQt5YfvFo16