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

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

docs/known-issues.md

@@ -238,9 +238,27 @@ fill-value item that did is fixed).
     before these checks, 16 and 9 before a VL type's stored element size
     was checked, which flags `cve-2024-32608`).
 - **Writer:**
-  - Nested groups beyond one level: path-like names are now refused, not
-    created.
-  - Dense attribute storage for attributes over 64 KiB.
+  - ~~Nested groups beyond one level: path-like names are now refused, not
+    created.~~ **Fixed 2026-09-26:** groups nest to any depth (path names
+    create intermediate groups, as h5py does), with soft, extra hard and
+    external links at any depth and optional creation-order tracking;
+    h5py, h5dump and `h5rs check --data` read them
+    (`crates/clawhdf5/tests/writer_groups_interop.rs`,
+    `crates/clawhdf5-tools/tests/h5rs_interop.rs`). Still missing: a group
+    with more than 65 535 links, or an object with more than 65 535 dense
+    attributes, is an error (the index is one B-tree leaf), and attribute
+    creation order is not tracked.
+  - ~~Dense link or attribute storage past 512 KiB of messages was written
+    unreadable (child indirect blocks of the fractal heap written as direct
+    blocks).~~ **Fixed 2026-09-26** (it affected 2.7.0 too): tested with
+    20 000 and 65 535 links and with 8 MB of dense attributes, read by
+    h5py, h5dump, `h5rs check` and clawhdf5, and h5py can add links to
+    such groups.
+  - ~~libhdf5 could not add a link to a group we wrote (no Group Info
+    message).~~ **Fixed 2026-09-26.**
+  - Huge fractal heap objects: in dense storage (more than 8 attributes on
+    an object, or more than 8 links in a group) one attribute or link
+    message over 65 515 bytes is an error.
   - Output that HDF5 1.8 can read.
   - A B-tree v2 chunk index larger than one leaf, so datasets with several
     unlimited dimensions are limited to 65 535 chunks.