fix(format): honour unknown-message flags the way libhdf5 does

The object header parser failed on an unknown message with flag bit 3
set and ignored bit 7. Per the spec, bit 3 means "fail if unknown and
the file is opened for writing" and bit 7 "fail if unknown, always".
The parser only reads, so it now ignores bit 3 (as libhdf5 does for a
read-only open) and refuses bit 7, in v1 headers, v2 headers and their
continuation chunks.

On libhdf5's conformance file tbogus.h5 (added as a fixture) we used to
refuse Dataset2 and open Dataset3; we now match libhdf5: Dataset1, 2, 4
and 5 open, Dataset3 is refused.

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

crates/clawhdf5-format/tests/writer_meta_tests.rs

@@ -563,3 +563,37 @@ fn slash_in_a_group_or_dataset_name_is_an_error() {
     let bytes = fw.finish().unwrap();
     header_at(&bytes, "g/c");
 }
+
+// ---- 7. unknown-message flags on read ----
+
+#[test]
+fn unknown_message_flags_follow_libhdf5_on_tbogus() {
+    // libhdf5's own test file (test/testfiles/tbogus.h5): datasets carrying
+    // an unknown message with various flags. libhdf5 (read-only) opens
+    // Dataset1, 2, 4 and 5 and refuses Dataset3 ("unknown message with 'fail
+    // if unknown' flag found"). We used to refuse Dataset2 (bit 3, which only
+    // applies when writing) and open Dataset3 (bit 7, fail always).
+    let bytes = include_bytes!("fixtures/tbogus.h5");
+    let sig = signature::find_signature(bytes).unwrap();
+    let sb = Superblock::parse(bytes, sig).unwrap();
+    for (name, readable) in [
+        ("Dataset1", true),
+        ("Dataset2", true),
+        ("Dataset3", false),
+        ("Dataset4", true),
+        ("Dataset5", true),
+    ] {
+        let addr = resolve_path_any(bytes, &sb, name).unwrap();
+        let parsed = ObjectHeader::parse(bytes, addr as usize, sb.offset_size, sb.length_size);
+        match parsed {
+            Ok(_) => assert!(readable, "{name} must be refused"),
+            Err(e) => {
+                assert!(!readable, "{name} must be readable, got {e:?}");
+                assert!(matches!(
+                    e,
+                    clawhdf5_format::error::FormatError::UnsupportedMessage(_)
+                ));
+            }
+        }
+    }
+}