docs: changelog and known issues for the plugin-filter review fixes

The short-decoding chunk (wrong data, pre-existing), the Blosc header
underflow (crash) and filter 32023 registration, each with its date and
what it changes; the conformance count is unchanged at 573 of 697.

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

CHANGELOG.md

@@ -31,6 +31,27 @@
   missing feature ("unsupported filter: 32026 (Blosc2, not implemented by
   clawhdf5)").
 - **Not implemented:** Blosc2 (32026) and ZFP (32013) remain a clear error.
+- **Wrong data: a chunk that decodes short read as zeros** (pre-existing, every
+  filter). HDF5 stores every chunk at the full chunk size, so a filter
+  pipeline that decodes to fewer bytes means a corrupt chunk; every chunk
+  reader (full, cached, selection, parallel, partial) padded it with zeros.
+  It is now an error naming the chunk ("chunk at [16] decoded to 16 bytes,
+  expected 32"), via the new `filters::decompress_chunk_exact`. libhdf5
+  returns the rest of such a chunk uninitialised, or fails when the filter
+  checks. A Blosc frame declaring no data for a non-empty chunk is an error
+  too. Legitimate edge chunks are unaffected (they are stored full-size,
+  filtered or not); conformance is unchanged at 573 of 697, with no file
+  changing class.
+- **Crash: a hostile Blosc chunk panicked** in builds with overflow checks
+  (debug builds, `cargo test`, `maturin develop`): a frame size below the
+  16-byte header underflowed. It is now an error. Every new decoder (LZF,
+  bitshuffle, bzip2, Blosc/BloscLZ) is fuzzed with random and mutated frames
+  in the unit tests.
+- **`register_filter(32023, ..)` works with the `pcodec` feature.** 32023 is
+  Granular BitRound's ID; the built-in entry there only reads clawhdf5
+  <= 2.7.0's pcodec chunks (filter name `"pcodec"`), so a registered codec now
+  handles every other chunk with that ID, and writes. It was refused as
+  "built in".
 
 ### Upgrade Notes
 - **HDF5 correctness audit (2026-09-25).** A sweep of 686 public files (the

docs/known-issues.md

@@ -140,7 +140,16 @@ fill-value item that did is fixed).
   697 ok). **Still open:** Blosc2 (32026 — hdf5plugin stores each chunk as a
   Blosc2 super-chunk frame, and n-D chunks as B2ND arrays) and ZFP (32013);
   both fail with an `UnsupportedFilter` error that names the filter, and
-  either can be plugged in with `filter_registry::register_filter`.
+  either can be plugged in with `filter_registry::register_filter` (32023,
+  Granular BitRound, too, since 2026-09-26 even with the `pcodec` feature).
+- **Wrong data: a chunk whose filters decode to fewer bytes than the chunk
+  read with zeros for the missing bytes** (any filter; found reviewing the plugin
+  filters). **Fixed 2026-09-26:** it is an error naming the chunk. A corrupt
+  chunk must never read as zeros. Unfiltered chunks are read at their stored
+  size and are not checked this way.
+- **Crash:** a hostile Blosc chunk (frame size below its header) panicked in
+  builds with overflow checks. **Fixed 2026-09-26**; the new decoders are
+  fuzzed in the unit tests.
 - **Header checks:** on 12 CVE datasets libhdf5 rejects a corrupt header and
   we read data anyway. We need stricter header checks.
 - **Writer:**