fix(format): resolve shared fill value messages instead of zero-filling

dataset_fill_value treated a shared Fill Value message as "no fill
value", so unwritten storage of a dataset whose fill value lives in the
file's shared-message (SOHM) heap read as zeros rather than its fill
value. libhdf5 shares fill values whenever the file has a SOHM index for
them.

- fill_value::dataset_fill_value_in follows the reference (another object
  header, or the SOHM heap); read_full_with_fill and the facade's
  selection read use it.
- dataset_fill_value, which has no file to follow a reference into, now
  returns UnresolvedSharedMessage for a shared message instead of None.
- shared_message::load_sohm_table / message_data_with_sohm load the SOHM
  table from the superblock extension on demand.
- parse_sohm_table skipped each index's leading version byte, reading
  every field one byte off; SOHM references could never resolve.

Fixture shared_fill_value.h5 (HDF5 2.0, gen_shared_fill.py): sohm_b read
[0,1,2,3,0,0,0,0] and now reads [0,1,2,3,-7,-7,-7,-7], as h5py does.

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

crates/clawhdf5-format/tests/writer_meta_tests.rs

@@ -597,3 +597,50 @@ fn unknown_message_flags_follow_libhdf5_on_tbogus() {
         }
     }
 }
+
+// ---- 8. shared fill value messages ----
+
+#[test]
+fn shared_fill_value_is_resolved_not_zero() {
+    // gen_shared_fill.py: HDF5 2.0 with a SOHM index for fill values, so each
+    // dataset's fill value message is a reference into the SOHM heap. It
+    // used to be read as "no fill value" (zeros) instead of -7.
+    let bytes = include_bytes!("fixtures/shared_fill_value.h5");
+    for (name, shared) in [
+        ("sohm_a", false),
+        ("sohm_b", true),
+        ("unwritten_a", false),
+        ("unwritten_b", true),
+    ] {
+        let (sb, oh) = header_at(bytes, name);
+        let msg = oh
+            .messages
+            .iter()
+            .find(|m| m.msg_type == MessageType::FillValue)
+            .unwrap();
+        assert_eq!(
+            clawhdf5_format::shared_message::is_shared(msg.flags),
+            shared,
+            "{name}: fixture layout"
+        );
+        if shared {
+            // Without the file the reference cannot be followed: an error,
+            // never a silent default.
+            assert_eq!(
+                clawhdf5_format::fill_value::dataset_fill_value(&oh.messages),
+                Err(clawhdf5_format::error::FormatError::UnresolvedSharedMessage)
+            );
+        }
+        assert_eq!(
+            clawhdf5_format::fill_value::dataset_fill_value_in(
+                bytes,
+                &oh.messages,
+                sb.offset_size,
+                sb.length_size
+            )
+            .unwrap(),
+            Some((-7i32).to_le_bytes().to_vec()),
+            "{name}"
+        );
+    }
+}