format: read extensible array chunk indexes over Storage

ExtensibleArrayHeader::parse_in reads the header in one read, and
read_extensible_array_chunks_in reads each index block, super block and
data block as a prefix read and then one window of the whole structure
(paged data blocks included); checksums and elements are checked in the
window with bounds errors reported as the whole-file checks did. The
&[u8] functions are wrappers. New test: an array with inline elements
and a data block, cut at every length and damaged in each structure,
reads identically through a read_at-only CountingStorage.

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

crates/clawhdf5-format/src/storage.rs

@@ -240,6 +240,16 @@ impl<'a> Window<'a> {
         })
     }
 
+    /// A whole in-memory file as one window (base 0).
+    #[cfg(test)]
+    pub fn whole(bytes: &'a [u8]) -> Self {
+        Window {
+            bytes: Cow::Borrowed(bytes),
+            base: 0,
+            file_len: bytes.len(),
+        }
+    }
+
     /// Check that `[rel, rel + needed)` (relative to `base`) is in the file.
     pub fn ensure(&self, rel: usize, needed: usize) -> Result<(), FormatError> {
         match rel.checked_add(needed) {