Merge branch 'feat/p2b-writer-btree-internal-nodes' into feat/p2b-scale

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -48,6 +48,47 @@
   with partial edge chunks, sparse datasets and fill values, and datasets
   larger than the chunk cache.
 
+### Writer: large dense indexes (2026-09-26)
+- **`track_order` orders attributes too, as h5py's `track_order=True`
+  does.** It tracked link creation order only, so h5py listed a tracked
+  object's attributes by name. A tracking object's header now has the
+  attribute creation order tracked and indexed flags, an Attribute Info
+  message with the next creation order (also for inline attributes), and a
+  creation order on each inline attribute message; dense attribute storage
+  gets a creation-order index (B-tree type 9). `FileWriter::track_order` /
+  `FileBuilder::track_order` now apply to datasets' attributes as well, and
+  `DatasetBuilder::track_order` sets it per dataset. Groups and datasets
+  that track order are written differently from before; others are
+  unchanged. More than 65 535 attributes on a tracking object is an error
+  (libhdf5's creation order counter is 2 bytes). The reader
+  (`attribute::extract_attributes*`) lists a tracking object's attributes
+  in creation order. Test `track_order_lists_attributes_in_creation_order`
+  (h5py lists, reads and extends them in "r+" mode, including libhdf5's
+  move from inline to dense storage).
+- **No more 65 535-record limit on the writer's v2 B-trees.** Dense link
+  storage (name index and creation-order index), dense attribute storage
+  and the chunk index of datasets with more than one unlimited dimension
+  were written as a single leaf, so a group with more than 65 535 links, an
+  object with more than 65 535 dense attributes, or such a dataset with more
+  than 65 535 chunks was an error. The writer now builds internal nodes to
+  any depth (`clawhdf5_format::btree_v2_write`), with node capacities and
+  child-pointer widths from the same arithmetic as libhdf5's
+  `H5B2__hdr_init` (shared with the reader, `btree_v2::node_info`) and
+  libhdf5's node sizes (512 bytes for dense storage, 2048 for chunks).
+  Indexes that fit the old one-leaf layout are written byte for byte as
+  before. New tests `crates/clawhdf5/tests/deep_btree_interop.rs` (100 000
+  links, 70 000 attributes, 200 000 chunks; h5py, h5dump, clawhdf5, and
+  h5py "r+" edits) and `check_files_with_deep_btrees` (`h5rs check`).
+- **Links and attributes whose name hashes collide are found by name.** The
+  dense name indexes (a group's links: B-tree v2 type 5; an object's
+  attributes: type 8) are ordered by the name's lookup3 hash and, when two
+  hashes are equal, by the name itself, as libhdf5 compares them. The writer
+  broke ties by insertion order, so libhdf5 could not open one of two
+  colliding names (`"k69209"` and `"k155448"` share hash `0x3a0b13e6`;
+  collisions are likely from about 77 000 names). Regression test
+  `names_whose_hashes_collide_are_found_by_name` in
+  `crates/clawhdf5/tests/writer_groups_interop.rs`.
+
 ### Concurrent reads (2026-09-26)
 - **Full reads of chunked datasets scale with threads again when rayon's
   pool has one thread.** Each full read handed its chunks to rayon to