Gate prefilter activation for small workloads

[la3lma]

Summary

• add a configurable activation threshold so both FastPath and legacy match engines skip the Aho-Corasick prefilter when pattern sets are below the tuned cutoff
• default the threshold to 7000 patterns with safe parsing and fallbacks for invalid values
• add a regression test demonstrating the latency reduction when prefiltering is bypassed for small workloads

Testing

• mvn -pl rmatch -Dtest=FastPathPrefilterThresholdTest test (fails: local toolchain lacks Java 25, so Maven cannot honor target release 25)

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Codex Task