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/error.rs

@@ -206,6 +206,10 @@ pub enum FormatError {
     /// wrong message count, contradictory message flags, a message of a
     /// class that cannot be shared flagged shareable, …
     InvalidObjectHeader(&'static str),
+    /// A datatype message libhdf5 refuses to decode (the reason is
+    /// libhdf5's own error text): size 0, bit fields outside the type,
+    /// an empty enum name, a compound member outside its compound, …
+    InvalidDatatype(String),
 }
 
 impl fmt::Display for FormatError {
@@ -453,6 +457,9 @@ impl fmt::Display for FormatError {
             FormatError::InvalidObjectHeader(why) => {
                 write!(f, "corrupt object header: {why}")
             }
+            FormatError::InvalidDatatype(why) => {
+                write!(f, "invalid datatype: {why}")
+            }
         }
     }
 }