fix(format): read v2 B-tree internal nodes with libhdf5's pointer widths

An internal node's child pointer is an address, the child's record count
and (below the first internal level) the child subtree's total record
count. libhdf5 (H5B2__hdr_init) encodes the record count in the width of
a leaf's maximum and the subtree total in the width of cum_max_nrec for
that depth, computed level by level from the node size. The reader
guessed 2 * leaf_max and leaf_max^depth, which agree at depth 2 but not
at depth 3: a 24 000-link group's name index has depth 3, its root's
pointers were read 3 bytes wide instead of 2, and listing failed with a
garbage heap offset.

Regression tests: dense_group_with_a_three_level_name_index (h5py writes
24 000 links; listing compared with h5py) and
subtree_capacity_matches_libhdf5.

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

docs/known-issues.md

@@ -77,8 +77,9 @@ the VDS item, which is marked.
 - **Groups and links:**
   - Groups with a user-defined link type (e.g. 187) cannot be listed.
   - Dense groups with more than about 22 000 links cannot be listed.
-    *Partly fixed 2026-09-25:* a link heap past its root block's direct rows
-    (child indirect blocks) is now read.
+    **Fixed 2026-09-25:** two bugs — fractal-heap child indirect blocks had
+    the wrong row count, and v2 B-tree internal nodes at depth 3+ were read
+    with the wrong pointer widths.
   - Soft links are left out of `datasets()`.
 - **Dense attributes:** a large attribute stored as a fractal-heap "huge"
   object makes every attribute on the object fail. This affects real NetCDF