fix(format): size fractal heap child indirect blocks by their row's span

A child indirect block in row r of a fractal heap's doubling table spans
that row's block size of heap space, so it has
log2(size) - log2(start_block_size * width) + 1 rows (libhdf5's
H5HF__dtable_size_to_rows). The reader used row - first_indirect_row + 1,
which undercounts, so every object stored past the root block's direct
rows (512 KiB with libhdf5's defaults) was unreachable: dense groups with
a few thousand long link names, or ~20 000 short ones, could not be listed.

Regression test: dense_group_whose_heap_outgrows_the_root_direct_rows
(h5py writes 2 500 links with 248-byte names; listing compared with h5py).

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

CHANGELOG.md

@@ -299,6 +299,12 @@
   flag; Fletcher32 ahead of deflate (NetCDF-4's order). Unknown-message flags
   follow libhdf5 (`tbogus.h5`): "fail if unknown" is refused, "fail if unknown
   and writing" is ignored by a reader.
+- `clawhdf5-format` reader — dense groups and attributes (links or
+  attributes kept in a fractal heap indexed by a v2 B-tree):
+  - A link heap larger than the root indirect block's direct rows (512 KiB
+    with libhdf5's defaults: a few thousand long link names, or ~20 000 short
+    ones) could not be listed: child indirect blocks were given the wrong
+    number of rows, so every link stored in one was unreachable.
 - `clawhdf5-format` writer — **files libhdf5 rejects or reads wrong:**
   - Extensible Array (one unlimited dimension): chunks from index 244 on were
     written but never indexed and read as 0, by libhdf5 and by us.