docs: plugin filters and the filter registry

CHANGELOG (Unreleased): LZF, bitshuffle, bzip2 and Blosc read and write
in pure Rust, their features, the ChunkOptions::plugin field (breaking for
struct-literal construction), the filter registry, the named
UnsupportedFilter message, and Blosc2/ZFP still unimplemented.
README: the clawhdf5-format feature table gains lzf (default),
bitshuffle, bzip2, blosc and plugin-filters, with how to write them and
what is not implemented; no speed claims. docs/known-issues.md: the audit's
filter gap is marked fixed 2026-09-26 for LZF/bitshuffle/bzip2/Blosc,
Blosc2 and ZFP still open. CLAUDE.md: the clawhdf5-filters row no longer
says "No Blosc". clawhdf5-format's crate docs list the new features.

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

docs/known-issues.md

@@ -133,7 +133,14 @@ fill-value item that did is fixed).
   - x87 long double and binary128 are refused.
   - N-Bit on 64-bit scale-offset data and some N-Bit parameter layouts fail.
 - **Filters:** blosc, blosc2, bitshuffle, bzip2, LZF and zfp are not
-  implemented.
+  implemented. **Fixed 2026-09-26** for LZF (default-on `lzf` feature),
+  bitshuffle, bzip2 and Blosc 1 (`bitshuffle`, `bzip2`, `blosc`, or
+  `plugin-filters` for all), read and write, pure Rust; h5ex_d_lzf,
+  h5ex_d_bshuf, h5ex_d_bzip2 and h5ex_d_blosc now read (conformance 573 of
+  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`.
 - **Header checks:** on 12 CVE datasets libhdf5 rejects a corrupt header and
   we read data anyway. We need stricter header checks.
 - **Writer:**