docs: changelog and known issues for the remaining conformance errors

Conformance on tank, conformance/run.sh --no-fetch (2026-09-26): 597 of
697 ok, 6 our-errors (4 corrupt objects HDF5 2.0 reads through a bug, the
Blosc2 and ZFP filters), 2 mismatches (the known h5py big-endian VL bug).
Closes the known-issues entries for metadata cache images,
cve-2024-32624, cve-2020-10810/10812, and unfiltered chunks of the wrong
size; the N-Bit / 64-bit scale-offset entry is recorded as not our bug.

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

CHANGELOG.md

@@ -2,6 +2,60 @@
 
 ## Unreleased
 
+### Remaining conformance errors (2026-09-26)
+Conformance on tank, `conformance/run.sh --no-fetch`: 597 of 697 files
+ok (575 before). Of the 6 our-errors left, 4 are corrupt data HDF5 2.0
+reads only through a bug (listed in `CONFORMANCE.md`), 2 are the Blosc2 and
+ZFP filters; the 2 mismatches are the known h5py big-endian VL bug.
+- **Metadata cache images are read.** A file written with a metadata cache
+  image keeps its metadata cache entries in an image block the superblock
+  extension points at, and libhdf5 reads them in place of the file's own
+  bytes; in `h5clear_mdc_image.h5` the root group exists only there, and
+  every reader failed with `InvalidObjectHeaderVersion(0)`. `File`,
+  `MmapFile` and `LazyFile` (and `h5rs`) now apply the image at open
+  (`clawhdf5_format::superblock_ext`), with libhdf5's checks. A file whose
+  image libhdf5 cannot load opens in libhdf5 but nothing in it can be read;
+  `File::open` refuses it.
+- **The superblock extension is decoded at open, as libhdf5 does:** a File
+  Space Info or Metadata Cache Image message libhdf5 cannot decode makes the
+  open fail (`cve-2020-10810`, `cve-2020-10812` were opened).
+  `FormatError::InvalidSuperblockExtension`, `InvalidCacheImage`.
+- **Dataset storage libhdf5 refuses at open is refused at open**
+  (`FormatError::InvalidDatasetStorage`, `data_read::check_dataset_storage`):
+  an element count times element size that overflows (`cve-2024-32624`
+  `/Dset_OBJREF` opened and reported its shape), contiguous storage past the
+  end of the file, compact data of the wrong size. An empty contiguous
+  dataset at a defined address, which clawhdf5 up to v2.7.0 wrote, still
+  opens.
+- **Wrong or missing data fixed:**
+  - a simple dataspace of rank 0 holds one element (it held 0;
+    `cve-2020-18494`), and contiguous storage larger than the dataset reads
+    (`cve-2024-32623`, `cve-2025-2309`; libhdf5 ignores the excess);
+  - scale-offset: the packed codes start at byte 21 whatever size the chunk
+    records for `minval`, a chunk with `minbits` 0 and a fill value is all
+    fill values, full-width `minbits` stores the elements as they are
+    (these decoded differently from libhdf5); E-scale is refused, as in
+    libhdf5; codes past the end of the chunk stay an error
+    (`cve-2025-2308`, where HDF5 2.0 reads past its buffer);
+  - shuffle uses its own parameter as the element size, as libhdf5 does
+    (`cve-2025-44905`);
+  - an unfiltered chunk the index records at other than the chunk's size is
+    refused (it read with zeros for the missing bytes; `cve-2025-44904`),
+    as is a chunk B-tree key with a non-zero element offset.
+- **Refused as libhdf5 refuses them:** a v1 group with an empty link name
+  fails its listing (`FormatError::InvalidLinkName`; lookups still work,
+  `cve-2021-46244`); dataspaces with more than 32 dimensions, a rank on a
+  scalar or null dataspace, or a dimension over its maximum
+  (`FormatError::InvalidDataspace`).
+- `ObjectHeader::object_class` classifies a header as libhdf5 does (a
+  dataset needs a datatype *and* a dataspace).
+- Conformance harness: user-defined links were listed as objects by the
+  reference, unopenable objects were not deduplicated, nested array types
+  were hashed wrong (`tarray3.h5`), and the attributes of objects h5py
+  cannot open were compared; all fixed. `CONFORMANCE.md` lists the corrupt
+  objects HDF5 2.0 reads through a bug (`bad_nbit_parms_walk.h5` among
+  them: libhdf5's own test now requires that read to fail).
+
 ### Concurrent reads (2026-09-26)
 - **Full reads of chunked datasets scale with threads again when rayon's
   pool has one thread.** Each full read handed its chunks to rayon to