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-tools/src/dtype.rs

@@ -135,6 +135,9 @@ pub fn short(dt: &Datatype) -> String {
         return f.short.into();
     }
     match dt {
+        Datatype::Complex { size, base_type } => {
+            short(&Datatype::complex_as_compound(*size, base_type))
+        }
         Datatype::FixedPoint {
             size,
             signed,
@@ -213,6 +216,9 @@ pub fn long(dt: &Datatype) -> String {
         return f.long.into();
     }
     match dt {
+        Datatype::Complex { size, base_type } => {
+            long(&Datatype::complex_as_compound(*size, base_type))
+        }
         Datatype::FixedPoint {
             size,
             signed,
@@ -492,6 +498,9 @@ fn string_ddl(
 /// hdf5-json type object.
 pub fn json(dt: &Datatype) -> J {
     match dt {
+        Datatype::Complex { size, base_type } => {
+            json(&Datatype::complex_as_compound(*size, base_type))
+        }
         Datatype::FixedPoint { .. } => {
             json!({"class": "H5T_INTEGER", "base": atomic_ddl(dt)})
         }
@@ -589,6 +598,7 @@ fn pad_json(p: &StringPadding) -> &'static str {
 /// Class name used to decide whether two datatypes can be compared.
 pub fn class(dt: &Datatype) -> &'static str {
     match dt {
+        Datatype::Complex { .. } => "compound",
         Datatype::FixedPoint { .. } => "integer",
         Datatype::FloatingPoint { .. } => "float",
         Datatype::Time { .. } => "time",

crates/clawhdf5-tools/src/value.rs

@@ -183,6 +183,11 @@ impl<'a> Decoder<'a> {
             return Value::Error("short element".into());
         };
         match dt {
+            Datatype::Complex { size, base_type } => self.decode(
+                &Datatype::complex_as_compound(*size, base_type),
+                b,
+                depth + 1,
+            ),
             Datatype::FixedPoint { .. } => match decode_int(dt, b) {
                 Some(v) => Value::Int(v),
                 None => Value::Bytes(b.to_vec()),