docs: dense storage fixes and the real limits of big groups

The changelog, known issues and README said a group holds up to 65 535
links while a group of about 17 000 was already unreadable. Record the
fixes (child indirect blocks, the next-block offset, the index leaf cap,
refusing oversized dense messages, hard-link memoisation, dataset
attribute overwrite) and the limits that remain true: 65 535 links or
dense attributes per object, and 65 515 bytes per dense message.

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

docs/known-issues.md

@@ -209,11 +209,20 @@ fill-value item that did is fixed).
     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 (its link index is one B-tree leaf) is an
-    error, and attribute creation order is not tracked.
+    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.**
-  - Dense attribute storage for attributes over 64 KiB.
+  - 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.