Comparing BCn texture decoders

PC GPUs use “BCn” texture compression formats (see “Understanding BCn Texture Compression Formats” by Nathan Reed or “Texture Block Compression in Direct3D 11” by Microsoft). While most of the interest is in developing BCn compressors (see “Texture Compression in 2020” post), I decided to look into various available BCn decompressors.
Why would you want that? After all, isn’t that done by the GPU, magically and efficiently? Normally, yes. Except if for “some reason” you need to access pixels on the CPU, or perhaps to use BCn data on a GPU that does not support it, or perhaps just to decode a BCn image in order to evaluate the compression quality/error.