fix(format): flag non-ASCII link names as UTF-8

The writer marked every link name ASCII, so a name such as "größe" was
stored as UTF-8 bytes under the ASCII character set (h5py reports cset 0
for it). Names that are not plain ASCII now carry the UTF-8 flag, as h5py
writes them; ASCII names are unchanged.

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

CHANGELOG.md

@@ -41,6 +41,9 @@
   `h5rs check` passes and `h5rs dump` equals h5dump
   (`crates/clawhdf5/tests/writer_groups_interop.rs`,
   `crates/clawhdf5-tools/tests/h5rs_interop.rs`).
+- **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.
 - **libhdf5 could not add links to groups we wrote.** h5py in `"r+"` mode
   failed with "Unable to create link (message type not found)" on every
   group `FileWriter` wrote: libhdf5 reads a group's Group Info message before