fix(format): refuse path-like group and dataset names

FileWriter writes the root group plus one level of groups; it has no way
to create intermediate groups. create_group("a/b") therefore stored a
single link literally named "a/b", which no HDF5 reader can resolve
(h5py: "component not found"). Nesting would mean restructuring the
writer's layout around a group tree, so for now finish() rejects any
group, dataset or external-link name that is empty, "." or contains '/'.
Attribute names may still contain '/'.

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

crates/clawhdf5-format/tests/writer_meta_tests.rs

@@ -524,3 +524,42 @@ fn h5py_reads_all_attributes_next_to_an_empty_string() {
     assert_eq!(out, r#"["", "héllo", ["", ""], 3]"#);
     h5dump_ok(&path);
 }
+
+// ---- 6. path-like names ----
+
+#[test]
+fn slash_in_a_group_or_dataset_name_is_an_error() {
+    // Measured: create_group("a/b") wrote one link literally named "a/b",
+    // which h5py cannot reach ("component not found"). The writer has no
+    // nested groups, so such names are refused.
+    let mut fw = FileWriter::new();
+    let mut g = fw.create_group("a/b");
+    g.create_dataset("c").with_f64_data(&[1.0]);
+    fw.add_group(g.finish());
+    assert!(fw.finish().is_err());
+
+    let mut fw = FileWriter::new();
+    fw.create_dataset("x/y").with_f64_data(&[1.0]);
+    assert!(fw.finish().is_err());
+
+    let mut fw = FileWriter::new();
+    let mut g = fw.create_group("g");
+    g.create_dataset("x/y").with_f64_data(&[1.0]);
+    fw.add_group(g.finish());
+    assert!(fw.finish().is_err());
+
+    for bad in ["", "."] {
+        let mut fw = FileWriter::new();
+        fw.create_dataset(bad).with_f64_data(&[1.0]);
+        assert!(fw.finish().is_err(), "{bad:?}");
+    }
+
+    // One level of groups still works, and '/' stays legal in attribute names.
+    let mut fw = FileWriter::new();
+    let mut g = fw.create_group("g");
+    g.create_dataset("c").with_f64_data(&[1.0]);
+    g.set_attr("m/s", AttrValue::I64(1));
+    fw.add_group(g.finish());
+    let bytes = fw.finish().unwrap();
+    header_at(&bytes, "g/c");
+}