fix(format): selections of v4 implicit-index chunked data no longer panic

read_raw_data_selection's chunked fallback (taken when partial_read
declines, e.g. a bounding box over half the dataset) handed the layout's
chunk dimensions, element-size dimension included, to
generate_implicit_chunks, which indexed past the dataset rank. It then
decoded the whole dataset regardless, so the enumeration is gone: the
arm decodes and extracts for every chunk index.

The new test reads small and large hyperslabs of all five v4 indexes
written by h5py and compares with h5py's values; it panicked before.

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

crates/clawhdf5-format/src/chunked_read.rs

@@ -510,6 +510,10 @@ fn collect_chunk_info_inner(
 ///
 /// Chunks are stored contiguously starting at `base_address`. No stored index;
 /// addresses are computed from the chunk position.
+///
+/// `chunk_dimensions` are the spatial chunk dimensions, one per entry of
+/// `dataset_dims` — not the layout message's list, which carries the element
+/// size as an extra last dimension.
 pub fn generate_implicit_chunks(
     base_address: u64,
     dataset_dims: &[u64],