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-tools/src/check.rs

@@ -916,7 +916,7 @@ impl Checker<'_> {
                 bad.push("has size 0".into());
             } else if !filtered
                 && let Some(cb) = chunk_bytes
-                && u64::from(c.chunk_size) != cb
+                && c.chunk_size != cb
             {
                 bad.push(format!(
                     "is {} bytes; an unfiltered chunk is {cb}",
@@ -933,7 +933,7 @@ impl Checker<'_> {
                     );
                 }
             }
-            self.extent(c.address, u64::from(c.chunk_size), path);
+            self.extent(c.address, c.chunk_size, path);
         }
         if reported > 50 {
             self.problem(

crates/clawhdf5-tools/src/info.rs

@@ -224,7 +224,7 @@ pub fn allocated_bytes(h5: &H5, info: &DsInfo) -> Result<u64> {
             let ds = info.ds.as_ref().map_err(Clone::clone)?;
             chunks(h5, layout, ds, dt)?
                 .iter()
-                .map(|c| u64::from(c.chunk_size))
+                .map(|c| c.chunk_size)
                 .sum()
         }
         DataLayout::Virtual { .. } => 0,