format: keep dataset_fill_value_in's &[u8] signature

Making dataset_fill_value_in generic over Storage broke callers that pass
an array (`include_bytes!`): a generic parameter does not unsize-coerce
`&[u8; N]`. It takes &[u8] again, as before this branch, and wraps the
new dataset_fill_value_from_storage(&dyn Storage, ..).

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

crates/clawhdf5-format/tests/storage_equivalence.rs

@@ -44,7 +44,7 @@ use clawhdf5_format::error::FormatError;
 use clawhdf5_format::extensible_array::{
     ExtensibleArrayHeader, read_extensible_array_chunks, read_extensible_array_chunks_in,
 };
-use clawhdf5_format::fill_value::dataset_fill_value_in;
+use clawhdf5_format::fill_value::{dataset_fill_value_from_storage, dataset_fill_value_in};
 use clawhdf5_format::fixed_array::{
     FixedArrayHeader, read_fixed_array_chunks, read_fixed_array_chunks_in,
 };
@@ -189,7 +189,7 @@ impl Walk<'_> {
         self.same(
             "fill value",
             &want,
-            &dataset_fill_value_in(self.storage, &header.messages, os, ls),
+            &dataset_fill_value_from_storage(self.st(), &header.messages, os, ls),
         );
 
         for msg in &header.messages {