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]>

docs/QUICKSTART.md

@@ -218,7 +218,8 @@ with clawhdf5.File("data.h5", "r+") as f:
 
 `'r+'` cannot create or delete datasets and groups, or delete attributes
 (`NotImplementedError`, nothing written). New files (`'w'`) take numeric
-arrays (`float64`, `float32`, `int64`, `int32`, `uint8`):
+arrays (`float64`, `float32`, `int64`, `int32`, `uint8`, and `complex64`/
+`complex128`, stored as h5py's compound `{r, i}`):
 
 ```python
 with clawhdf5.File("new.h5", "w") as f:

docs/known-issues.md

@@ -174,6 +174,15 @@ wrong data.
   decoded, and external references are an error (object references
   decode). A multi-dimensional numeric attribute is returned as a flat
   array (its shape is not reported; `AttrValue::Raw` carries the shape).
+  HDF5 2.0's native complex type (class 11) is read as the equivalent
+  `{r, i}` compound: values are right (`Dataset::read_complex_f64`, and
+  numpy complex in Python), but `raw_datatype()`, `dtype()`, `h5rs
+  ls`/`dump` and the browser reader show a compound where h5dump 2.x
+  prints `H5T_COMPLEX_IEEE_F64LE`, so `h5rs dump` of such a file does not
+  match h5dump 2.x (h5dump 1.14 cannot read it at all). Writing class 11
+  (added 2026-09-28) is opt-in (`with_native_complex_f64_data`,
+  `make_native_complex_f64_type`); the Python bindings write complex
+  arrays as h5py's compound only.
 - **Metadata cache images** (read since 2026-09-26) differ from libhdf5 in
   that: libhdf5 fails only the first metadata read of an image it cannot
   load and then reads the file's own (possibly stale) metadata, where we