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/README.md

@@ -100,8 +100,11 @@ f.remote_stats   # {'requests': ..., 'bytes_fetched': ..., 'hits': ..., ...}
 
 `clawhdf5.File(path, "w")` with `create_dataset(name, data=array,
 chunks=..., compression="gzip")`, `create_group` and `attrs[...] = ...`
-writes `float64`, `float32`, `int64`, `int32` and `uint8` arrays; the file is
-written on `close()`.
+writes `float64`, `float32`, `int64`, `int32`, `uint8`, `complex64` and
+`complex128` arrays; the file is written on `close()`. Complex arrays are
+stored as h5py stores them, a compound `{r, i}` that every libhdf5 reads
+(not HDF5 2.0's native complex type, which only libhdf5 2.0+ reads; the
+Rust API writes that on request).
 
 ## Editing a file in place