conformance: report a cache image libhdf5 cannot load where it does

libhdf5 loads a metadata cache image when it first reads metadata (the
root group), not at open, so for cve-2025-6269-1..4 and cve-2025-6516 (all
corrupt images) h5py opens the file and fails on "/". The probe reported
the image's error as an open error, which made those files our-errors;
it now records it on the root object, where h5py reports it. File::open
still refuses such a file outright: nothing in it can be read.

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

conformance/probe/src/main.rs

@@ -712,18 +712,35 @@ fn main() {
             return;
         }
     };
-    // libhdf5 decodes the superblock extension at open, and loads a
-    // metadata cache image over the file's own metadata.
-    let view = match guarded(|| {
-        clawhdf5_format::superblock_ext::metadata_view(hdf5, &sb).map_err(e)
-    }) {
-        Ok(v) => v,
+    // libhdf5 decodes the superblock extension at open (File::open does
+    // the same), and loads a metadata cache image over the file's own
+    // metadata. It loads the image only when it first reads metadata — the
+    // root group — so a file whose image it cannot load still opens and
+    // every object fails; File::open refuses such a file outright. The
+    // probe records the image's error where libhdf5 reports it.
+    use clawhdf5_format::superblock_ext;
+    let ext = match guarded(|| superblock_ext::read_superblock_extension(hdf5, &sb).map_err(e)) {
+        Ok(x) => x,
         Err(msg) => {
             top.insert("open_error".into(), Value::String(msg));
             println!("{}", Value::Object(top));
             return;
         }
     };
+    let mut image_error = None;
+    let view = match ext.and_then(|x| x.cache_image) {
+        None => None,
+        Some(loc) => match guarded(|| {
+            superblock_ext::apply_cache_image(hdf5, loc, sb.offset_size, sb.length_size)
+                .map_err(e)
+        }) {
+            Ok(v) => Some(v),
+            Err(msg) => {
+                image_error = Some(msg);
+                None
+            }
+        },
+    };
     let hdf5: &[u8] = view.as_deref().unwrap_or(hdf5);
     top.insert("superblock_version".into(), json!(sb.version));
     let ctx = Ctx {
@@ -752,6 +769,9 @@ fn main() {
         let mut rec = Map::new();
         rec.insert("path".into(), Value::String(p.clone()));
         let r = guarded(|| {
+            if let Some(msg) = &image_error {
+                return Err(msg.clone());
+            }
             let h = ctx.header(addr)?;
             Ok(h)
         });