fix(format): refuse a chunk layout whose element size is not the datatype's

A chunked layout records the element size as its last dimension, and
libhdf5 refuses a dataset whose datatype has another size
(H5D__chunk_set_sizes: "stored datatype size in chunk layout does not
match datatype description"). clawhdf5 ignored the recorded size and
read the chunks anyway, for v3 and v4 layouts. The check runs on every
chunked read (read_chunked_data*, read_raw_data_selection) and compares
against the stored size: a variable-length element is 4 + offset size
+ 4 bytes, not Datatype::type_size's 16.

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

CHANGELOG.md

@@ -335,7 +335,9 @@
     dimension, a chunk rank that does not match the dataspace, a chunk of
     4 GiB or more indexed by a v1 B-tree (layout version 3 or earlier;
     0x80000000-sized chunks hung the reader — layout versions 4 and 5 allow
-    larger chunks, and HDF5 2.0 writes them), and v1 B-tree
+    larger chunks, and HDF5 2.0 writes them), an element size in the
+    layout that differs from the datatype's stored size (the chunks were
+    laid out with the wrong element size), and v1 B-tree
     chunk keys whose offsets are not multiples of the chunk dimensions,
     including the keys that only bound a node
     (`chunked_read::collect_chunk_info_checked`).