A selection read (a hyperslab or points covering at most half the
dataset) decoded each chunk it overlaps into fresh buffers, one per
filter stage; it now uses the thread's chunk-decoding scratch like the
full readers. Covered by tests/chunked_read_paths_interop.rs (small
hyperslabs and points over every filter and type) and the partial-read
equivalence tests.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>