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),
     }
 }
 

crates/clawhdf5/tests/writer_groups_interop.rs

@@ -592,3 +592,21 @@ fn track_order_lists_members_in_creation_order() {
     assert_eq!(out, r#"["aaa", ["soft", "aaa"], 21]"#);
     h5dump_ok(&path);
 }
+
+#[test]
+fn non_ascii_names_are_utf8() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let mut b = FileBuilder::new();
+    b.create_dataset("größe/wert").with_i32_data(&[1]);
+    let path = write(&dir, "utf8.h5", b);
+    let out = h5py(
+        &path,
+        "with h5py.File(path, 'r') as f:\n\
+         \x20   l = f.id.links.get_info('größe'.encode())\n\
+         \x20   print(json.dumps([list(f), list(f['größe']), l.cset], ensure_ascii=False))",
+    );
+    assert_eq!(out, r#"[["größe"], ["wert"], 1]"#);
+    let f = File::open(&path).unwrap();
+    assert_eq!(f.dataset("größe/wert").unwrap().read_i32().unwrap(), [1]);
+}