libhdf5 allocates an implicit index's chunks for the whole maximum
extent and places chunk `scaled` at its row-major position in the
maximum chunk grid (H5D__none_idx_get_addr, max_down_chunks). The reader
used the current grid, so a dataset below its maximum shape with more
chunk columns at its maximum read other chunks' values from the second
chunk row on (h5py early allocation, fixed maxshape).
generate_implicit_chunks_in_grid takes the maximum dimensions;
generate_implicit_chunks keeps its signature (grid = current extent).
Regression: implicit_chunks_use_the_maximum_grid here, and
implicit_index_below_its_maximum_reads_like_libhdf5 (h5py file) with the
editor tests.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
