fix(format): read unmapped VDS elements as the virtual dataset's fill value

Elements of a virtual dataset that no mapping supplies (unmapped regions,
a missing source file, a missing source dataset) read as 0 instead of the
fill value libhdf5 returns — silent wrong data for any VDS created with a
non-zero fillvalue (read-matrix cases 0471/0472: -1 and 7 read as 0). A
missing source dataset was an error; libhdf5 reads it as fill.

Move VDS assembly into a new vds module following H5Dvirtual.c:
vds::read_virtual_dataset takes the dataset's fill value and a
VdsFileResolver that can refuse a name, and reports how many elements were
unmapped. Sources are read with their own fill value, and a source whose
datatype differs from the virtual dataset's is an error (libhdf5 converts).
File passes the dataset's fill value, resolves source names against the
virtual file's directory, and refuses names that leave it with an error
instead of reading them as fill. read_selection on a VDS goes through the
same fill-aware path.

The raw-read API (read_raw_data_full*) has no fill value, so it now errors
for a VDS with unmapped elements instead of guessing zeros.

Tests: vds_interop::vds_unmapped_regions_read_as_fill_value (external,
same-file, missing file/dataset, sparse source with its own fill, int
fill; earliest and latest format) and
vds_source_outside_directory_is_an_error_not_fill, both against h5py;
integration_test::v4_virtual_dataset_raw_api_refuses_to_guess_the_fill_value.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/lib.rs

@@ -100,6 +100,7 @@ pub mod signature;
 pub mod superblock;
 pub mod symbol_table;
 pub mod type_builders;
+pub mod vds;
 pub mod vl_data;
 
 #[cfg(feature = "provenance")]