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_write.rs

@@ -2263,7 +2263,7 @@ mod tests {
         for i in 0..n {
             let (offsets, chunk) = extract_chunk(&data, &shape, &chunks, 2, i);
             assert_eq!(chunk.len(), 2 * 3 * 2 * 2);
-            for (e, pair) in chunk.chunks_exact(2).enumerate() {
+            for (e, pair) in chunk.as_chunks::<2>().0.iter().enumerate() {
                 let c = [e / 6, (e / 2) % 3, e % 2];
                 let g: Vec<u64> = (0..3).map(|d| offsets[d] + c[d] as u64).collect();
                 let expect = if (0..3).all(|d| g[d] < shape[d]) {
@@ -2272,7 +2272,7 @@ mod tests {
                 } else {
                     [0, 0]
                 };
-                assert_eq!(pair, expect, "chunk {i} element {e}");
+                assert_eq!(*pair, expect, "chunk {i} element {e}");
             }
         }
         // Data shorter than the shape: the missing elements stay zero.