fix(format): refuse datatypes libhdf5 refuses to decode

Datatype::parse now makes the checks of libhdf5's H5O__dtype_decode_helper
and fails with InvalidDatatype (libhdf5's own error text) instead of
decoding a corrupt type:

- size 0 ("invalid datatype size"), for every class;
- integer bit offset/precision outside the type, or precision 0;
- float sign/exponent/mantissa outside the type, empty, or overlapping;
  normalization 3; bit 6 without bit 0 from version 3;
- compound with no members, a member outside the compound, a duplicate
  name, or a member overlapping an earlier one;
- enum whose size differs from its base type's, or an empty member name;
- array of more than 32 dimensions or with a zero-sized one (v1 compound
  array members now say so rather than InvalidDatatypeVersion);
- opaque tag length that is not a multiple of 8.

Bit 6 of a version-1/2 float's class bits used to be read as VAX order,
byte-swapping values; libhdf5 ignores it before version 3, and so does
this now.

Only checks HDF5 2.0 (h5py 3.16) makes are added: newer libhdf5 also
checks bit fields, the variable-length kind and array sizes, but h5py
opens files that fail those, so they are left out. Each check was
confirmed against h5py by corrupting a file it wrote.

The conformance probe now decodes committed datatypes, as h5py's f[name]
does. Conformance (cached corpus, tank): 570 ok, unchanged. Objects
libhdf5 refuses that clawhdf5 used to read: cve-2016-4332-mtime (/cmpnd),
cve-2017-17508, cve-2024-32616 (/type1), cve-2024-32618, cve-2026-34734,
bad_compound.h5 (/cmpnd, /dataset); eight more that already failed now
fail with libhdf5's reason (e.g. cve-2024-29163 "mantissa range out of
bounds").

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

crates/clawhdf5-format/src/attribute.rs

@@ -573,7 +573,8 @@ mod tests {
 
     /// Build an f64 LE datatype message.
     fn build_f64_dt() -> Vec<u8> {
-        let mut buf = build_dt_header(1, 1, [0x00, 0x00, 0x02], 8);
+        // Sign bit 63 (bits 8-15 of the class bits).
+        let mut buf = build_dt_header(1, 1, [0x20, 63, 0x00], 8);
         let mut props = [0u8; 12];
         props[2..4].copy_from_slice(&64u16.to_le_bytes()); // bit_precision
         props[4] = 52; // exp_location