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/lib.rs

@@ -161,6 +161,10 @@ pub(crate) enum DatasetData {
     I64(Vec<i64>),
     I32(Vec<i32>),
     U8(Vec<u8>),
+    /// numpy `complex64`, `[re, im]` pairs, written as h5py does.
+    C64(Vec<[f32; 2]>),
+    /// numpy `complex128`, `[re, im]` pairs, written as h5py does.
+    C128(Vec<[f64; 2]>),
 }
 
 /// Specification for a dataset to be written.
@@ -281,6 +285,14 @@ pub(crate) fn apply_dataset_spec(
         DatasetData::U8(v) => {
             db.with_u8_data(v);
         }
+        // h5py's compound `{r, i}`, not HDF5 2.0's native complex type: it
+        // is what h5py writes (3.16 included) and every libhdf5 can read it.
+        DatasetData::C64(v) => {
+            db.with_complex_f32_data(v);
+        }
+        DatasetData::C128(v) => {
+            db.with_complex_f64_data(v);
+        }
     }
     if !spec.shape.is_empty() {
         db.with_shape(&spec.shape);
@@ -314,9 +326,19 @@ pub(crate) fn extract_numpy_data(
         "int64" => DatasetData::I64(flat.extract::<Vec<i64>>()?),
         "int32" => DatasetData::I32(flat.extract::<Vec<i32>>()?),
         "uint8" => DatasetData::U8(flat.extract::<Vec<u8>>()?),
+        // A 1-D complex array viewed as floats is its (re, im) parts in order.
+        "complex64" => {
+            let parts: Vec<f32> = flat.call_method1("view", ("<f4",))?.extract()?;
+            DatasetData::C64(parts.as_chunks::<2>().0.to_vec())
+        }
+        "complex128" => {
+            let parts: Vec<f64> = flat.call_method1("view", ("<f8",))?.extract()?;
+            DatasetData::C128(parts.as_chunks::<2>().0.to_vec())
+        }
         _ => {
             return Err(PyErr::new::<pyo3::exceptions::PyTypeError, _>(format!(
-                "unsupported numpy dtype: {dtype_str}; expected float64, float32, int64, int32, or uint8"
+                "unsupported numpy dtype: {dtype_str}; expected float64, float32, int64, int32, \
+                 uint8, complex64 or complex128"
             )));
         }
     };