#3164 Fix 16-bit binary PBM sample byte order

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

The binary PGM and PPM formats store 16-bit samples most significant byte first. The decoder and encoder read and wrote them in native little-endian order, so every wide sample was byte swapped. This corrects the swizzled colors reported for 16-bit PPM images.

• Reverse the sample byte order in BinaryDecoder and BinaryEncoder.
• Fix integer division in the max pixel value upscale factor.
• Add exact-value, reference-decoder, upscale, and round-trip tests.
• Add a 16-bit PPM test image and its Magick-anchored reference PNG.
• Document the binary decoder and encoder in full.