feat(format): follow soft links; explicit errors for external links and external raw data

- Path resolution follows soft links in both old-style (symbol table, cache
  type 2) and new-style (compact and dense Link message) groups: absolute and
  relative targets, links to groups, links through links, with a depth limit
  so a link cycle is NestingDepthExceeded rather than a hang. A dangling link
  reports the target it could not find. Previously every soft link was
  PathNotFound.
- An external link is FormatError::ExternalLinkUnsupported { filename,
  object_path } instead of a misleading PathNotFound.
- Message 0x0007 (External Data Files) is now a known MessageType, and a
  dataset carrying it is FormatError::ExternalDataFilesUnsupported. Such a
  dataset has no data address in this file, so it would otherwise be read as
  "never written" and answered with fill values — wrong data, no error.
- Dense link iteration is shared between hard-link listing and the new
  symbolic-link lookup; entry listing behaviour is unchanged.
- h5py interop test for both libver settings.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/clawhdf5-format/src/fill_value.rs

@@ -165,6 +165,15 @@ pub fn read_full_with_fill<E: From<FormatError>>(
     length_size: u8,
     read: impl FnOnce() -> Result<Vec<u8>, E>,
 ) -> Result<Vec<u8>, E> {
+    // A dataset with external raw data also has no data address in this
+    // file. It is NOT unallocated — its values live elsewhere — so it must
+    // never be answered with the fill value.
+    if messages
+        .iter()
+        .any(|m| m.msg_type == MessageType::ExternalDataFiles)
+    {
+        return Err(FormatError::ExternalDataFilesUnsupported.into());
+    }
     let fill = dataset_fill_value(messages)?;
     if !has_storage(layout) {
         return Ok(filled_dataset(dataspace, elem_size, fill.as_deref())?);