shuffle_decompress — on the read path of every compressed dataset, since
shuffle is applied automatically before compression — was the naive
`result[i * es + j] = data[j * n + i]`: a multiply and two bounds checks per
byte. It now interleaves fixed-width arrays of byte planes for element sizes
2/4/8/16 (bounds checks hoisted, vectorisable), with a chunked generic
fallback. The write-side shuffle was already optimised; this was the asymmetry
the survey flagged. Modest wall-clock effect now that decode is parallel
(chunked+deflate full read ~70 -> ~66 ms). Round-trip test over element sizes
1-24 and several lengths.
Co-Authored-By: Claude Fable 5.1 <[email protected]>
