fix(format): dataspaces and contiguous storage as libhdf5 reads them

- A simple dataspace of rank 0 holds one element in libhdf5 (the product
  of no dimensions; h5py reads it as shape ()). num_elements() said 0, so
  cve-2020-18494's /dset1 failed with DataSizeMismatch { expected: 0 }.
- A contiguous dataset whose storage is larger than its elements reads:
  libhdf5 reads the elements from the start of the storage and ignores the
  rest (H5D__contig_check checks only that they fit in the file). We
  required the sizes to be equal, so the scalar /Dset1 of cve-2024-32623
  and cve-2025-2309 (240 bytes of storage for one int) failed. Storage too
  small for the elements is still an error. data_read::contiguous_read_len
  is the rule, used by every contiguous read path.
- Dataspace::parse refuses what H5O__sdspace_decode refuses: more than 32
  dimensions, a rank on a scalar or null dataspace, a dimension larger
  than its maximum (new FormatError::InvalidDataspace).

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

crates/clawhdf5-format/src/error.rs

@@ -226,6 +226,10 @@ pub enum FormatError {
     /// A link libhdf5 refuses to list: a symbol-table entry with an empty
     /// name ("invalid link name"). Listing the group fails, as in libhdf5.
     InvalidLinkName,
+    /// A dataspace message libhdf5 refuses to decode (the reason is
+    /// libhdf5's own error text): more than 32 dimensions, a rank on a
+    /// scalar or null dataspace, a dimension larger than its maximum.
+    InvalidDataspace(&'static str),
 }
 
 impl fmt::Display for FormatError {
@@ -500,6 +504,9 @@ impl fmt::Display for FormatError {
             FormatError::InvalidLinkName => {
                 write!(f, "invalid link name: a group entry has an empty name")
             }
+            FormatError::InvalidDataspace(why) => {
+                write!(f, "invalid dataspace: {why}")
+            }
         }
     }
 }