fix(format): a group attribute set again replaces the earlier value

Setting a group or root attribute twice wrote two attribute messages with
the same name, and h5py read back the first value: set_attr("w", 1) then
set_attr("w", "two") read as 1. The later value now replaces the earlier
one, as `attrs[name] = v` does in h5py, including when a group is merged
from two builders.

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

CHANGELOG.md

@@ -41,6 +41,11 @@
   `h5rs check` passes and `h5rs dump` equals h5dump
   (`crates/clawhdf5/tests/writer_groups_interop.rs`,
   `crates/clawhdf5-tools/tests/h5rs_interop.rs`).
+- **A group attribute set twice read back as its first value.** Setting a
+  group (or root) attribute again wrote a second attribute message with the
+  same name, and h5py returned the first value. The later value now replaces
+  the earlier one, as `attrs[name] = v` does in h5py — also when a group is
+  merged from two builders.
 - **Non-ASCII link names were marked ASCII.** A group or dataset name such as
   `größe` was written with the ASCII character set flag (h5py reported
   `cset` 0 for it); it is now flagged UTF-8, as h5py writes it.