docs: CHANGELOG for the Blosc2 allocation bounds

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

CHANGELOG.md

@@ -21,6 +21,21 @@
   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