The sample.heif was 8736x5856 (2.5MB), causing each processing
operation to take 17-38s in CI. Resized to 1432x960 (224KB) and added
a 15-minute timeout to the test job.
- Add ppm/pgm/pbm to CLI_DECODED_FORMATS with Sharp-first, ImageMagick fallback
- Create sample.qoi test fixture (10x10 solid color)
- Add QOI to format-matrix test FORMAT_SAMPLES
- Mark PPM/PGM/PBM as needsCliDecoder in tests (Sharp doesn't support them natively)