feat: write complex numbers, incl. HDF5 2.0 native complex (class 11)

- Datatype::Complex serializes class 11 version 5 byte-identically to
  libhdf5 2.2.0; containers holding it are written as version 5.
- DatasetBuilder::with_complex_f32/f64_data (h5py's {r, i} compound,
  default) and with_native_complex_f32/f64_data (class 11, opt-in);
  make_(native_)complex_f32/f64_type for attributes.
- Dataset::read_complex_f64/f32 read either form.
- Python create_dataset accepts complex64/complex128 (compound form).
- Parsing unchanged: class 11 still surfaces as {r, i}.
- Tests vs h5py 3.16 / libhdf5 2.0.0 and h5dump 2.2.0; docs.

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

crates/clawhdf5-py/src/edit.rs

@@ -47,6 +47,7 @@ fn not_implemented(what: impl std::fmt::Display) -> PyErr {
 /// `edit_helpers._convert_array`), or why they cannot be written.
 pub(crate) fn category(dt: &Datatype) -> PyResult<&'static str> {
     match dt {
+        Datatype::Complex { .. } => Ok("complex"),
         Datatype::FixedPoint { .. } => Ok("int"),
         Datatype::FloatingPoint { .. } if crate::convert::is_ieee_float(dt) => Ok("float"),
         // Read as a wider IEEE float; writing would need the reverse
@@ -107,7 +108,9 @@ fn check_exact(dt: &Datatype) -> PyResult<()> {
         Datatype::Compound { members, .. } => {
             members.iter().try_for_each(|m| check_exact(&m.datatype))
         }
-        Datatype::Array { base_type, .. } => check_exact(base_type),
+        Datatype::Array { base_type, .. } | Datatype::Complex { base_type, .. } => {
+            check_exact(base_type)
+        }
         Datatype::String {
             padding: StringPadding::NullPad,
             ..