Merge branch 'feat/p2b-blosc2' into feat/p2b-scale

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -89,6 +89,41 @@
   `names_whose_hashes_collide_are_found_by_name` in
   `crates/clawhdf5/tests/writer_groups_interop.rs`.
 
+### Blosc2 (2026-09-26)
+- **Blosc2 (filter 32026) reads, in pure Rust.** Files written with
+  hdf5plugin's `Blosc2` failed with `UnsupportedFilter`. New feature
+  `blosc2` (`clawhdf5-format` and `clawhdf5`, included in `plugin-filters`)
+  decodes the Blosc2 contiguous frame hdf5-blosc2 stores per chunk, the
+  B2ND arrays it uses for chunks of 2 or more dimensions (blocks gathered
+  back into C order), Blosc2 chunks with their special values (zeros, NaN,
+  uninitialised, one repeated value), and the shuffle, bit-shuffle, delta
+  and truncate-precision filters, over the BloscLZ, LZ4/LZ4HC, Zlib and
+  Zstandard codecs shared with Blosc 1. Read only: there is no Blosc2
+  encoder. Dictionaries, lazy chunks, variable-length blocks, user-defined
+  codecs and registered Blosc2 filters (e.g. bytedelta) are errors;
+  uninitialised chunks read as zeros. Tested against h5py 3.16 +
+  hdf5plugin 7.1 (every codec, filter and level 0-9; 1- to 5-D chunks with
+  partial edge chunks; every integer width and f4/f8; datasets of zeros,
+  one value and NaN; Fletcher32 before Blosc2) and against frames from
+  python-blosc2 4.13.1 for what hdf5plugin never writes
+  (`crates/clawhdf5-format/tests/fixtures/blosc2/`); the decoder is
+  fuzzed. Conformance: 576 of 697 files ok (was 575) — h5ex_d_blosc2.
+- **A crafted Blosc2 chunk cannot allocate more than a few times its HDF5
+  chunk size.** Found in review before release: the sizes a frame declares
+  sized the decoder's buffers. A 173-byte frame whose offsets chunk claimed
+  2 GiB was decoded in full for a 1 MiB chunk; an empty chunk allocated its
+  declared block size twice (about 1 GiB); a B2ND chunk was decoded whole
+  with its padding (up to 16x the chunk); and a Zstandard stream's declared
+  window (up to 100 MiB) was reserved as the decoder was reused, which also
+  affected Blosc 1 and bitshuffle with Zstandard. Now the offsets chunk is
+  capped at the chunk size, block sizes are clamped to the chunk, B2ND
+  blocks are placed as they are decoded (padding is never held), and the
+  Zstandard window is capped at twice the stream's output (at least
+  128 KiB). A B2ND chunk may no longer be larger than its array, which
+  hdf5-blosc2 never writes. `tests/blosc2_alloc_bounds.rs` measures peak
+  allocation for these frames and for 45,000 fuzzed ones: at most 6x the
+  chunk size, twice the input and 2 MiB of Zstandard state.
+
 ### 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
@@ -459,6 +494,7 @@
   missing feature ("unsupported filter: 32026 (Blosc2, not implemented by
   clawhdf5)").
 - **Not implemented:** Blosc2 (32026) and ZFP (32013) remain a clear error.
+  (Blosc2 reads since the `blosc2` feature, see above.)
 - **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