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/tests/raw_fetch_bounds.rs

@@ -151,7 +151,7 @@ fn crafted() -> (Vec<u8>, Chunked, Vec<ChunkInfo>) {
         // v1 B-tree key (size, filter mask, offsets + 0) then the child
         // address.
         let mut pat = Vec::new();
-        pat.extend_from_slice(&c.chunk_size.to_le_bytes());
+        pat.extend_from_slice(&(c.chunk_size as u32).to_le_bytes());
         pat.extend_from_slice(&c.filter_mask.to_le_bytes());
         // The key holds one offset per dimension plus the element offset
         // (0); `offsets` may or may not list that last one.