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]>

crates/clawhdf5-format/src/file_writer.rs

@@ -220,6 +220,16 @@ fn add_refcount(w: &mut ObjectHeaderWriter, refcount: u32) {
     }
 }
 
+/// The character set a link name is written with: UTF-8 when it is not
+/// plain ASCII, as h5py writes it.
+fn name_charset(name: &str) -> CharacterSet {
+    if name.is_ascii() {
+        CharacterSet::Ascii
+    } else {
+        CharacterSet::Utf8
+    }
+}
+
 /// The Link message for `link`, whose group and dataset targets are at the
 /// given addresses (indexed as in the writer tree).
 fn link_message(link: &writer_tree::Link, group_addrs: &[u64], ds_addrs: &[u64]) -> LinkMessage {
@@ -242,7 +252,7 @@ fn link_message(link: &writer_tree::Link, group_addrs: &[u64], ds_addrs: &[u64])
         name: link.name.clone(),
         link_target,
         creation_order: link.creation_order,
-        charset: CharacterSet::Ascii,
+        charset: name_charset(&link.name),
     }
 }