⚡ Bolt: Unroll AVX2 match length comparison to 256 bytes

[404Setup]

Optimized match_len_avx2 in src/compress/matchfinder.rs by unrolling the comparison loop to process 256 bytes per iteration (previously 128 bytes).

This reduces the overhead of _mm256_testz_si256 (which checks for any mismatch) by performing it half as often for long matches. The implementation accumulates XOR differences using _mm256_or_si256 (which is cheap) before performing the expensive test.

Correctness was verified with cargo test compress::matchfinder, including explicit tests for AVX2 and tail handling. The performance improvement targets long match scenarios (e.g. repetitive data) which are common in compression workloads.

Note: An earlier attempt to optimize adler32 AVX2 by breaking dependency chains resulted in a regression due to register spilling, so that change was reverted.

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PR created automatically by Jules for task 11708908719364509432 started by @404Setup

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