feat(format): bzip2 filter (307), read and write, pure Rust

hdf5plugin's BZip2 failed with UnsupportedFilter(307). The new `bzip2`
feature decodes the single bzip2 stream H5Zbzip2.c stores, bounded by the
chunk size (a truncated stream is an error, not short data), and encodes
at block size cd_values[0] (DatasetBuilder::with_bzip2(level)). It uses the
bzip2 crate's default backend, libbz2-rs-sys, a pure-Rust port of
libbzip2: `cargo tree` shows no cc/cmake, and nothing is compiled from C.

Interop: hdf5plugin writes block sizes 9, 1 and 5+shuffle over the
12-case matrix, read byte for byte; ours at 9 (shuffled) and 1 (not)
reads back through hdf5plugin. Both fail with the decoder removed.

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

crates/clawhdf5-format/Cargo.toml

@@ -25,6 +25,9 @@ pco = { version = "1.0", optional = true }
 # Pure-Rust Zstandard, for the plugin filters that embed zstd (bitshuffle,
 # blosc). The `zstd` feature (filter 32015) links libzstd instead.
 ruzstd = { version = "0.9", optional = true }
+# bzip2 with its default backend, libbz2-rs-sys: a pure-Rust port of
+# libbzip2 (no C is compiled, despite the -sys name).
+bzip2 = { version = "0.6", optional = true }
 
 [dev-dependencies]
 half = { workspace = true }
@@ -64,6 +67,8 @@ pcodec = ["dep:pco"]
 lzf = []
 # Bitshuffle (32008), with its LZ4 and Zstandard modes.
 bitshuffle = ["lz4_flex", "ruzstd"]
+# bzip2 (307).
+bzip2 = ["dep:bzip2", "std"]
 
 [[bench]]
 name = "parallel_decompress_bench"