feat(py): h5py-style reads of only the selected elements, GIL released

ds[key] read the whole dataset and sliced it in numpy, and knew six
dtypes. Keys (ints, positive-step slices, Ellipsis, one increasing index
list, compound field names) now map onto hyperslab selections, and the
facade's read_selection bytes become the numpy buffer without a copy
(PyArray::from_vec viewed as the dtype). dtype mapping follows h5py for
all integer/IEEE float widths and byte orders, bool, enum, complex, fixed
and variable-length strings, vlen sequences, opaque, array types and
(nested, padded) compounds; anything it cannot describe exactly is a
TypeError. Attributes return what h5py returns; groups and files gain
the rest of the h5py mapping interface. Reads run under py.detach.

tests/test_read_vs_h5py.py compares >500 reads with h5py 3.16 on an
h5py-written file, checks errors match, that a damaged chunk outside the
selection is never touched, and 8 threads reading at once.

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

crates/clawhdf5-py/tests/conftest.py

@@ -0,0 +1,18 @@
+"""Shared fixtures for the clawhdf5 Python binding tests."""
+
+import os
+
+import pytest
+
+
+@pytest.fixture(scope="session")
+def h5py():
+    """h5py, or a skip — unless CLAWHDF5_REQUIRE_INTEROP=1, which makes a
+    missing h5py a failure (as for the Rust interop suites)."""
+    try:
+        import h5py as mod
+    except ImportError:
+        if os.environ.get("CLAWHDF5_REQUIRE_INTEROP") == "1":
+            pytest.fail("h5py is required (CLAWHDF5_REQUIRE_INTEROP=1) but not importable")
+        pytest.skip("h5py not installed")
+    return mod