Merge branch 'feat/p2-writer-groups-links' into feat/p2-perf-coverage

# Conflicts:
#	CHANGELOG.md
#	crates/clawhdf5-tools/tests/h5rs_interop.rs

CHANGELOG.md

@@ -139,6 +139,97 @@
   `crates/clawhdf5/tests/vl_data_interop.rs`). `read_vl_bytes` now also
   treats address 0 as null whatever the length, as `VlResolver` does.
 
+### Writer: groups and links (2026-09-26)
+- **Nested groups, to any depth.** `FileWriter`/`FileBuilder` wrote the root
+  group plus one level, and refused path-like names. Now a name may be a path
+  (`create_dataset("a/b/x")`, `create_group("a/b")`, a leading `/` at the
+  root) and missing intermediate groups are created, as h5py does; groups
+  also nest through the new `GroupBuilder::create_group`/`add_group`. A group
+  added at a path that already holds a group is merged into it (h5py's
+  `require_group`); a name used twice otherwise, an empty or `"."`
+  component (`"a//b"`, `"a/"`) or an absolute path below the root is an
+  error. Datasets, attributes, dense attribute storage and dense link
+  storage work at every level.
+- **Soft, hard and external links at any depth:** `add_soft_link(name,
+  target)` (h5py's `SoftLink`; the target may dangle),
+  `add_hard_link(name, target)` (h5py's `f[name] = f[target]`; the target
+  path is resolved when the file is written, may go through other hard
+  links, and a missing target, a soft link on the way or a cycle of
+  hard-link paths is an error) and `add_external_link`, on `FileWriter`,
+  `FileBuilder` and `GroupBuilder`. An object with several hard links gets
+  an Object Reference Count message, so libhdf5 can delete one of the links
+  without freeing the object.
+- **Link creation order:** `track_order(true)` on a `GroupBuilder`, or on
+  `FileWriter`/`FileBuilder` for every group that does not set its own,
+  tracks and indexes link creation order (h5py's `track_order=True`): the
+  Link Info message carries the flags, each link its order, and a dense
+  group a creation-order B-tree (type 6). h5py then lists members in
+  insertion order. Attribute creation order is not tracked.
+- A group holds at most 65 535 links (its link index is one B-tree leaf),
+  and in a group of more than 8 links (dense storage) each link message
+  must be at most 65 515 bytes (one fractal heap block; huge heap objects
+  are not written); more is an error. Measured at the limit: 65 535 links
+  with 100-byte names (a 7 MB heap) read in h5py, h5dump and clawhdf5, and
+  h5py can add to the group. `GroupBuilder`'s fields changed (they were
+  crate-private); `FinishedGroup` is unchanged for callers.
+- Files that use one level of groups and no new link kinds are laid out as
+  before: byte-identical to the writer with the Group Info fix below
+  (compared on simple, mixed dense/chunked/compact/external-link and paged
+  files). Tests: h5py and clawhdf5 read the same
+  tree (every path, attribute and value) from a 5-level file; soft, hard,
+  external and cyclic hard links; 10 000, 20 000 and 65 535 links in one
+  group, with and without creation order; libhdf5 adding and deleting links
+  in our groups;
+  `h5rs check` passes and `h5rs dump` equals h5dump
+  (`crates/clawhdf5/tests/writer_groups_interop.rs`,
+  `crates/clawhdf5-tools/tests/h5rs_interop.rs`).
+- **Big dense groups and attribute sets were unreadable.** The fractal heap
+  holding dense links or attributes wrote every doubling-table row as
+  direct blocks, but past the 512 KiB the root's direct blocks hold, rows
+  are child indirect blocks, and libhdf5 and `h5rs check` read them as
+  such: a group with 20 000 links of 20-byte names was written without
+  error and h5py could not list it ("incorrect metadata checksum"); 150
+  dense attributes of up to 56 KB could not be opened. This was in 2.7.0
+  too. The heap writer now writes child indirect blocks, nested as deep as
+  needed. Found on the way: an object bigger than the next block's space
+  was cut off (it now goes in the first block big enough), and h5py adding
+  a link to a heap over 64 KiB overwrote its first block (the header's
+  next-block offset was 0).
+- **h5py crashed adding a link to a group of more than about 47 700
+  links** (35 000 with creation order tracked). The link index leaf's node size gave libhdf5 room for more than
+  65 535 records, which overflows the leaf's 2-byte count. The node is now
+  capped at 65 535 records. Dense attributes use the same index builder:
+  more than 65 535 on one object used to be written with the count modulo
+  65 536, and are now an error.
+- **A dense link or attribute message over 65 515 bytes** (e.g. a soft link
+  with a long target in a group of more than 8 links) was written cut off,
+  and libhdf5 could not list the group ("object overruns end of direct
+  block"). It is now an error.
+- **Chained hard links took exponential time to resolve.** A hard-link
+  target going through other hard links resolved them again on every path
+  through them: 26 links whose targets each named the previous one twice
+  took 46 s. Each hard link is now resolved once, and a cycle is reported
+  by the link's name.
+- **A dataset attribute set twice read back as its first value**, as for
+  groups below (h5py listed the name twice). The later value now replaces
+  the earlier one; a hand-set attribute named like a provenance attribute
+  is replaced by the computed one.
+- **A group attribute set twice read back as its first value.** Setting a
+  group (or root) attribute again wrote a second attribute message with the
+  same name, and h5py returned the first value. The later value now replaces
+  the earlier one, as `attrs[name] = v` does in h5py — also when a group is
+  merged from two builders.
+- **Non-ASCII link names were marked ASCII.** A group or dataset name such as
+  `größe` was written with the ASCII character set flag (h5py reported
+  `cset` 0 for it); it is now flagged UTF-8, as h5py writes it.
+- **libhdf5 could not add links to groups we wrote.** h5py in `"r+"` mode
+  failed with "Unable to create link (message type not found)" on every
+  group `FileWriter` wrote: libhdf5 reads a group's Group Info message before
+  inserting a link, and none was written. Every group now carries one
+  (version 0, default thresholds: 6 more bytes per group header, so files
+  are not byte-identical to earlier versions). Regression test:
+  `crates/clawhdf5/tests/writer_groups_interop.rs`.
+
 ### Plugin filters (2026-09-26)
 - **LZF, bitshuffle, bzip2 and Blosc read and write, in pure Rust.** Files
   written by h5py with `compression="lzf"`, or with hdf5plugin's