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]>

crates/clawhdf5-netcdf4/src/cf.rs

@@ -142,6 +142,7 @@ fn get_fill_value(attrs: &HashMap<String, AttrValue>, key: &str) -> Option<FillV
         Some(AttrValue::String(s)) => Some(FillValue::String(s.clone())),
         Some(AttrValue::F64Array(arr)) if !arr.is_empty() => Some(FillValue::Float(arr[0])),
         Some(AttrValue::I64Array(arr)) if !arr.is_empty() => Some(FillValue::Int(arr[0])),
+        Some(AttrValue::U64Array(arr)) if !arr.is_empty() => Some(FillValue::UInt(arr[0])),
         _ => None,
     }
 }
@@ -155,6 +156,10 @@ fn get_valid_range(attrs: &HashMap<String, AttrValue>) -> Option<(f64, f64)> {
         Some(AttrValue::I64Array(arr)) if arr.len() >= 2 => {
             return Some((arr[0] as f64, arr[1] as f64));
         }
+        // Unsigned variables (NC_UBYTE..NC_UINT64) carry unsigned attributes.
+        Some(AttrValue::U64Array(arr)) if arr.len() >= 2 => {
+            return Some((arr[0] as f64, arr[1] as f64));
+        }
         _ => {}
     }