Stacking feat/libver-v18 under feat/huge-chunks sent every chunked dataset
of a file with a 1.8 low bound to a version-1 B-tree, whose key holds a
32-bit chunk size. As in libhdf5, such a chunk now takes layout version 5
whatever the low bound, and a high bound below 2.0 is FormatError::LibverBound.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>