decompress_chunk_exact_with decodes a chunk into a DecodeScratch the
caller keeps between chunks, instead of a new Vec per chunk and per filter
stage. Deflate inflates into a kept buffer with a reset (not rebuilt)
inflater, shuffle interleaves into the other buffer, and Fletcher32 checks
and drops its checksum in place (on the stored bytes when it is the first
filter undone). Other filters go through the registry as before. Output
and errors are those of decompress_chunk_exact; a unit test checks that
for every pipeline shape and filter mask with one reused scratch.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
