edit: record the maximum before resizing a chunked dataset that has none

FileEditor::resize (on main since PR #18, a4c2ace) scrambled the values of
a chunked dataset whose dataspace records no maximum dimensions when it
shrank it: clawhdf5's writer stores such a dataspace for every chunked
dataset created without a maxshape, the maximum is then the current
dimensions, and the Fixed Array index linearises chunks by the maximum, so
patching only the current dimensions moved every chunk after the first
row. h5py, h5dump and our reader all read the wrong values; the dataset
could not grow back either.

libhdf5 never writes such a dataspace (H5S_set_extent_simple records the
maximum, equal to the dimensions when none is given); reading one,
H5S_extent_get_dims reports the current dimensions as the maximum and
H5S_set_extent checks against none, so its own H5Dset_extent scrambles
such a file the same way. The editor now records the maximum libhdf5 would
have written (the dimensions the index was built with) before changing the
current ones, moving the grown dataspace message in the header when it
must. The writer records the maximum of every chunked dataset too, so
h5py can resize what clawhdf5 writes (the pinned file hashes of three
no-maxshape cases in plugin_filters_interop change by 8 bytes a dimension).

Tests: edit_resize_interop.rs (a 2.7.0-written fixture, new FileBuilder
files and h5py files through shrinks, zero extents and growth, against a
model with our reader and h5py; h5py resizing a FileBuilder file), and in
test_edit.py resizes checked against a numpy model, independently of h5py.

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

crates/clawhdf5-format/src/file_writer.rs

@@ -130,6 +130,22 @@ pub(crate) fn build_chunked_dataset_oh(
     fill_message: &[u8],
     refcount: u32,
 ) -> Result<Vec<u8>, FormatError> {
+    // libhdf5 records every simple dataspace's maximum dimensions (the
+    // dimensions themselves when none are given, `H5S_set_extent_simple`).
+    // Without them libhdf5 takes the maximum to be the current dimensions,
+    // so a resize by libhdf5 (h5py's `Dataset.resize`) would also change
+    // the maximum a Fixed Array chunk index is laid out by, and move every
+    // chunk already written.
+    let recorded;
+    let ds = if ds.space_type == DataspaceType::Simple && ds.max_dimensions.is_none() {
+        recorded = Dataspace {
+            max_dimensions: Some(ds.dimensions.clone()),
+            ..ds.clone()
+        };
+        &recorded
+    } else {
+        ds
+    };
     let mut w = ObjectHeaderWriter::new();
     w.add_message_with_flags(MessageType::Datatype, dt.serialize(), 0x01);
     w.add_message(MessageType::Dataspace, ds.serialize(LENGTH_SIZE));