fix(format): refuse a local heap whose free list leaves the heap

libhdf5 walks a local heap's free list when it loads the heap's data and
refuses the heap ("bad heap free list") when a free block starts or ends
outside the data segment, or links to offset 0. We never looked at the
free list, so a damaged old-style group listed names read from the broken
heap: once the user block of cve-2021-36977.h5 was applied, its root
listed eight garbage names where libhdf5 fails.

LocalHeap::validate_free_list (new) mirrors H5HL__fl_deserialize, with a
cycle bound, and accepts H5HL_FREE_NULL (1) or an all-ones head as the
end of the list. Like libhdf5 it runs when the first name is needed, not
on parse, so an empty group with a damaged heap still lists as empty
(cve-2018-13871.h5, cve-2024-29166.h5, gh-4431-poc-03.h5 keep matching
h5py).

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

docs/known-issues.md

@@ -86,6 +86,11 @@ the VDS item, which is marked.
   - 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.
   - Soft links are left out of `datasets()`.
+  - **Wrong data (found while fixing user blocks):** an old-style group whose
+    local-heap free list points outside the heap listed garbage names where
+    libhdf5 refuses the heap. **Fixed 2026-09-25**
+    (`InvalidLocalHeapFreeList`, checked when a name is first read, as
+    libhdf5 does).
 - **Dense attributes:** a large attribute stored as a fractal-heap "huge"
   object makes every attribute on the object fail. This affects real NetCDF
   files (`issue671.nc`).