Huge chunks: selection tests with fill values, LZ4 over 256 MiB, wasm32 check

A selection of a chunked dataset with a fill value is compared with the
full read in every index, through a map and through positioned reads. An
LZ4 chunk larger than 256 MiB is bounded by the chunk size, not refused.
The wasm package test reads the 4 GiB-chunk fixture and gets a clean
error in every index.

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

crates/clawhdf5-format/src/chunked_read.rs

@@ -1491,7 +1491,7 @@ pub fn list_chunks_for_read_in<S: Storage + ?Sized>(
         let chunk_bytes = checked_chunk_byte_len(&chunk_dims, elem_size)?;
         if let Some(c) = chunks
             .iter()
-            .find(|c| c.address != u64::MAX && c.chunk_size as usize != chunk_bytes)
+            .find(|c| c.address != u64::MAX && c.chunk_size != chunk_bytes as u64)
         {
             return Err(FormatError::ChunkedReadError(format!(
                 "incorrect chunk size returned from index for unfiltered chunk at {:?}: \
@@ -2470,7 +2470,7 @@ mod tests {
         // Entries: key[i], child[i] pairs, then final key
         for chunk in chunks {
             // Key: chunk_size(4) + filter_mask(4) + ndims offsets
-            buf.extend_from_slice(&chunk.chunk_size.to_le_bytes());
+            buf.extend_from_slice(&(chunk.chunk_size as u32).to_le_bytes());
             buf.extend_from_slice(&chunk.filter_mask.to_le_bytes());
             for d in 0..ndims {
                 let off = if d < chunk.offsets.len() {