fix(py): index lists of padded compounds no longer return uninitialised padding

np.concatenate copies structured dtypes field by field into np.empty, so
the padding of ds[[0, 3, 6]] held process memory. The runs' bytes are
joined in Rust, whole elements at a time, before anything becomes numpy:
the padding is the file's bytes (h5py's) and the result is still a view
of the Rust buffer. The h5py comparisons now compare every byte of
structured values; the new test failed on the padding before.

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

crates/clawhdf5-py/tests/test_read_vs_h5py.py

@@ -212,14 +212,19 @@ def assert_same(ours, theirs, what=""):
         if theirs.dtype == object:
             for a, b in zip(ours.ravel(), theirs.ravel()):
                 assert_same(a, b, what)
-        elif theirs.dtype.names is None and theirs.dtype.kind == "V":
-            assert ours.tobytes() == theirs.tobytes(), what
+        elif theirs.dtype.kind == "V":
+            # Structured and opaque: every byte, padding included (h5py's
+            # padding is zero; uninitialised memory there would leak).
+            if theirs.dtype.names is not None:
+                np.testing.assert_array_equal(ours, theirs, err_msg=what)
+            assert ours.tobytes() == theirs.tobytes(), f"{what}: bytes differ"
         else:
             np.testing.assert_array_equal(ours, theirs, err_msg=what)
     elif isinstance(theirs, np.generic):
         assert ours.dtype == theirs.dtype, what
         if theirs.dtype.names is not None:
             np.testing.assert_array_equal(np.asarray(ours), np.asarray(theirs), err_msg=what)
+            assert ours.tobytes() == theirs.tobytes(), f"{what}: bytes differ"
         else:
             assert ours == theirs or (ours != ours and theirs != theirs), what
     else:
@@ -511,3 +516,18 @@ def test_a_library_panic_is_an_ordinary_exception():
         clawhdf5._panic_for_test()
     except Exception:  # noqa: BLE001 - the point of the test
         pass
+
+
+def test_compound_padding_bytes_match_h5py(pair):
+    """Every byte of a padded compound, padding included, is h5py's, for
+    index lists with many runs as well as slices. Joining the runs with
+    np.concatenate left the padding uninitialised: process memory ended up
+    in tobytes()."""
+    ours, theirs, _ = pair
+    keys = [[0, 3, 6], [1, 2, 5, 9], [0, 2, 4, 6, 8], slice(None), slice(1, 9, 3), 4, [9]]
+    for name in ["cmp/padded", "cmp/padded_chunked"]:
+        for _ in range(20):  # garbage varies between runs; zeros do not
+            for key in keys:
+                assert ours[name][key].tobytes() == theirs[name][key].tobytes(), f"{name}[{key!r}]"
+    for key in [(slice(None), [0, 2]), ([0, 2, 3], slice(None)), ([1, 3], 1)]:
+        assert ours["cmp/nested_2d"][key].tobytes() == theirs["cmp/nested_2d"][key].tobytes(), key