chunked writer: chunks of 4 GiB or more never use the v1 B-tree; need high bound V200

Stacking feat/libver-v18 under feat/huge-chunks sent every chunked dataset
of a file with a 1.8 low bound to a version-1 B-tree, whose key holds a
32-bit chunk size. As in libhdf5, such a chunk now takes layout version 5
whatever the low bound, and a high bound below 2.0 is FormatError::LibverBound.

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

crates/clawhdf5-format/src/file_writer.rs

@@ -1517,7 +1517,9 @@ impl FileWriter {
     /// (virtual datasets, a paged file, the 1.12 reference types, native
     /// complex numbers). With a low bound of 1.8 and a later high bound
     /// such objects are written in the newer format, as libhdf5 writes them;
-    /// the rest of the file stays readable by 1.8.
+    /// the rest of the file stays readable by 1.8. A chunk of 4 GiB or more
+    /// always takes HDF5 2.0's version-5 layout (never a version-1 B-tree)
+    /// and so needs a high bound of at least [`LibVer::V200`].
     ///
     /// A low bound above the high bound makes [`Self::finish`] fail.
     pub fn libver_bounds(&mut self, low: LibVer, high: LibVer) -> &mut Self {
@@ -1830,6 +1832,7 @@ impl FileWriter {
                     dummy_cursor,
                     d.maxshape.as_deref(),
                     low,
+                    high,
                 )?;
                 dummy_cursor += result.data_bytes.len() as u64;
                 let oh = build_chunked_dataset_oh(
@@ -1991,6 +1994,7 @@ impl FileWriter {
                     base_address,
                     d.maxshape.as_deref(),
                     low,
+                    high,
                 )?;
                 cursor2 += result.data_bytes.len();
                 let oh = build_chunked_dataset_oh(