⚡ Bolt: [performance improvement] Optimize PatternSearcher.PrefixReject

[nt153133]

💡 What: Optimized PrefixReject in PatternSearcher.cs to use a direct 8-byte ulong unaligned memory read even when the prefix length is shorter than 8 bytes, masking out the unused bytes. A fallback slow loop is still retained for bounds safety at the very end of the searched span.

🎯 Why: When short byte patterns were passed in to Search / SearchMany that didn't have an 8-byte minimum prefix, PrefixReject fell back to manually reading each byte in a slow loop and bit-shifting them into an integer. This added overhead to the hot loop, which executes on nearly every index in the search span. Because cp.PrefixMask ignores extraneous bits past prefLen, it's totally safe to read up to 8 bytes and apply the bitmask, as long as an out-of-bounds error is avoided.

📊 Impact: Expected ~20-22% performance improvement on short pattern searches.

🔬 Measurement: Measured by creating a 10MB test byte array and running 100 SearchMany iterations over various short patterns, resulting in elapsed time dropping from ~753 ms down to ~586 ms.

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PR created automatically by Jules for task 16152097668367746085 started by @nt153133

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