DataLayout::resolve_vds_mappings_in reads the global heap collection
holding a virtual dataset's mappings through GlobalHeapCollection::
parse_in; resolve_vds_mappings is a wrapper. The rest of the data layout
module parses message payloads and does not read the file.
New test: the virtual dataset of vds_same_file.h5 resolves to the same
mappings through a read_at-only CountingStorage, in two reads.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>