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]>

docs/known-issues.md

@@ -100,13 +100,19 @@ name padding, whereas v2 keeps the 8-byte name padding and has no array fields.
 
 ## Attributes with unsupported datatypes are silently dropped
 
-**Status:** open.
+**Status:** fixed 2026-09-19.
 
-**Summary:** `Dataset::attrs()` / `Group::attrs()` in the `clawhdf5` facade return
-only attributes convertible to `AttrValue`. An attribute with, e.g., a compound
-datatype is omitted from the map with no error or indication that it exists.
-Planned: surface these as an explicit `AttrValue` variant (raw bytes + datatype)
-or an error, as part of the "no silent skips" robustness work.
+**Summary:** `Dataset::attrs()` / `Group::attrs()` returned only attributes
+convertible to `AttrValue` and omitted the rest without any indication — every
+Python `bool` (an HDF5 enum), complex, compound and reference attributes. Unsigned
+64-bit arrays were also cast to `I64Array`, turning values above `i64::MAX`
+negative.
+
+**Fix:** booleans decode as 0/1 integers, `AttrValue::U64Array` keeps unsigned
+arrays unsigned, and `AttrValue::Raw { datatype, shape, data }` carries any other
+attribute verbatim. Both new variants are writable. Still lossy: a
+multi-dimensional numeric attribute is returned as a flat array (its shape is
+not reported).
 
 ## B-tree v2 chunk index (layout v4, index type 5) is not supported