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/error.rs

@@ -117,6 +117,17 @@ pub enum FormatError {
     /// A message is marked shared but was parsed without access to the file,
     /// so the reference to the real message could not be followed.
     UnresolvedSharedMessage,
+    /// The dataset's raw data is stored in external files (External Data
+    /// Files message), which this reader does not follow.
+    ExternalDataFilesUnsupported,
+    /// The path goes through an external link (a link into another file),
+    /// which this reader does not follow.
+    ExternalLinkUnsupported {
+        /// The file the link points into.
+        filename: String,
+        /// The object path within that file.
+        object_path: String,
+    },
     /// Invalid SOHM table version.
     InvalidSohmTableVersion(u8),
     /// Invalid SOHM table signature (expected "SMTB").
@@ -310,6 +321,18 @@ impl fmt::Display for FormatError {
             FormatError::InvalidSharedMessageVersion(v) => {
                 write!(f, "invalid shared message version: {v}")
             }
+            FormatError::ExternalLinkUnsupported {
+                filename,
+                object_path,
+            } => write!(
+                f,
+                "path goes through an external link to {object_path} in {filename}, which is \
+                 not supported"
+            ),
+            FormatError::ExternalDataFilesUnsupported => write!(
+                f,
+                "dataset raw data is stored in external file(s), which is not supported"
+            ),
             FormatError::UnresolvedSharedMessage => write!(
                 f,
                 "message is shared but no file data was available to resolve it"