feat: attrs() reports every attribute; unsigned arrays stay unsigned

attrs() silently omitted any attribute whose datatype had no AttrValue variant
— including every Python bool, which h5py stores as an enum — plus complex,
compound and reference attributes, and cast unsigned 64-bit arrays to
I64Array so values above i64::MAX came back negative.

- numpy/h5py-style booleans (an enum of exactly FALSE=0 / TRUE=1 over an
  integer base) decode as I64 / I64Array of 0/1.
- AttrValue::U64Array keeps unsigned arrays unsigned. Behaviour change: an
  unsigned array attribute no longer arrives as I64Array; the netCDF-4 CF
  helpers (_FillValue, valid_range) and the Python bindings handle it.
- AttrValue::Raw { datatype, shape, data } carries any other attribute
  verbatim (also used when a value fails to decode as its declared type), so
  the attribute list is always complete. Decodable with data_read against the
  datatype; Python receives {"dtype", "shape", "data"}.
- Both new variants are writable, so attributes round-trip between files.
  h5py interop tests cover reading 13 attribute kinds and h5py reading back a
  compound and a u64 attribute written by clawhdf5.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

CHANGELOG.md

@@ -41,6 +41,20 @@
   object_path }` (was `PathNotFound`), and a dataset whose raw data lives in
   external files (message 0x0007, now a known `MessageType`) is
   `ExternalDataFilesUnsupported` (it would otherwise read as fill values).
+- **`attrs()` no longer drops attributes.** Any attribute whose datatype had
+  no `AttrValue` variant was omitted with no error — including every Python
+  `bool` (h5py stores `attrs["flag"] = True` as an enum), complex numbers,
+  compound values and object references. Now:
+  - numpy/h5py-style booleans (an enum of exactly `FALSE`=0 / `TRUE`=1) decode
+    as `I64` / `I64Array` of 0/1;
+  - new `AttrValue::U64Array` keeps unsigned arrays unsigned (they were cast to
+    `I64Array`, so values above `i64::MAX` came back negative). **Behaviour
+    change:** code matching `I64Array` for an unsigned attribute must also
+    match `U64Array` (the netCDF-4 CF helpers and Python bindings do);
+  - new `AttrValue::Raw { datatype, shape, data }` carries everything else
+    verbatim, decodable with `clawhdf5_format::data_read` against `datatype`.
+  Both new variants are writable, so an attribute can be copied between files
+  unchanged. Python receives `Raw` as `{"dtype", "shape", "data"}`.
 - All of the above are covered by h5py interop tests under both default and
   `libver='latest'` bounds, compared against h5py's own readback.