clawhdf5: FileEditor modifies existing files in place

New clawhdf5::FileEditor opens an HDF5 file (h5py-written at any libver,
HDF5 2.0 format included, or clawhdf5-written) under an exclusive flock
and changes only what an edit touches:
- write_selection/write_all/write_values: compact, contiguous (also
  late-allocated) and chunked datasets, any selection. Chunks are decoded,
  updated and re-encoded; a filtered chunk that no longer fits moves to the
  end of the file unless it is the file's last structure, which grows in
  place. New chunks go into v1 B-tree, Extensible Array (paged data blocks
  included), Fixed Array and single-chunk indexes, created on first use.
- resize: grow chunked datasets up to maxshape.
- set_attr: add/replace compact attributes, in a NIL slot or a new
  continuation chunk.
Each edit is planned in an in-memory image and refused whole
(Error::Unsupported) when any part is unsupported (v2 B-tree / implicit
new chunks, shrinking, vlen/reference data, dense or order-tracked
attributes, cache images, paged/persistent free space). Commit writes and
syncs new space before patching existing bytes. Layout v5 (HDF5 2.0)
array indexes use 8-byte filtered chunk sizes, as libhdf5 does.

Error gains Unsupported/InvalidArgument/Locked and is #[non_exhaustive];
the Python bindings map them. build_attr_message is public.

Tests (h5py, h5dump, h5rs check --data after every round; h5py r+
afterwards): appends crossing EA super/data blocks and B-tree splits, the
same B-tree node counts and EA statistics as libhdf5 for the same writes
(in order, reversed and shuffled; paged blocks), every layout and chunk
index overwritten under random selections, attributes to continuation
chunks, random operations against a model, refused edits leave the file
byte-identical, locking.

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

crates/clawhdf5-format/src/type_builders.rs

@@ -296,7 +296,8 @@ impl EnumTypeBuilder {
 
 // ---- Attribute helper ----
 
-pub(crate) fn build_attr_message(name: &str, value: &AttrValue) -> AttributeMessage {
+/// The attribute message the writers store for `value` under `name`.
+pub fn build_attr_message(name: &str, value: &AttrValue) -> AttributeMessage {
     match value {
         AttrValue::F64(v) => AttributeMessage {
             name: name.to_string(),