[bzip2] Stream decompressor output and export seed corpus

[carrerasdarren-cell]

Summary

• stream decompressor output through a fixed 4 KiB scratch buffer instead of limiting output to twice the compressed input size
• bound valid output processing at 16 MiB and stop if the decompressor makes no progress
• write the existing decompression seed archive to $OUT so OSS-Fuzz retains it

Rationale

BZ2_bzBuffToBuffDecompress currently receives an output capacity of 2 * input_size. Valid bzip2 streams can expand far beyond that ratio, so the target returns BZ_OUTBUFF_FULL before exercising the rest of the stream. All three upstream quick-test samples exceed the current limit:

Seed	Compressed	Decompressed	Ratio
sample1.bz2	32,348	98,696	3.05x
sample2.bz2	73,732	212,340	2.88x
sample3.bz2	235	120,244	511.68x

A deterministic 1 MiB zero-filled input compresses to 45 bytes. The old 90-byte output buffer returns BZ_OUTBUFF_FULL (-8); the streaming path reaches BZ_STREAM_END (4) after producing all 1,048,576 bytes.

The build script also creates bzip2_decompress_target_seed_corpus.zip in the container working directory after cd bzip2, rather than in $OUT. The archive is therefore absent from the build artifacts.

Verification

• python3 infra/helper.py build_fuzzers bzip2 --sanitizer address --clean
• python3 infra/helper.py build_fuzzers bzip2 --sanitizer undefined
• python3 infra/helper.py check_build bzip2 --sanitizer undefined
• replayed the exported 34-file seed corpus under UBSan with no failures
• replayed a 408-file evolved corpus under Linux ASan/UBSan with no failures

For a deterministic -runs=0 replay of the same exported corpus, the old target initialized at 450 edges / 1,344 features; the streaming target initialized at 464 edges / 1,574 features.

[github-actions]

carrerasdarren-cell is a new contributor to projects/bzip2. The PR must be approved by known contributors before it can be merged. The past contributors are: OwenSanzas, arthurscchan, hunsche, fanquake, swirsz, cvediver, Dor1s, sylvestre, posidron (unverified), bshastry (unverified), tysmith (unverified)