fix: isMatchDFA concurrent safety — prefilter rejection only (#137)

[kolkov]

Summary

isMatchDFA prefilter candidate loop called e.dfa.SearchAtAnchored concurrently from RunParallel. Shared lazy DFA is NOT thread-safe for concurrent lazy state construction. On ARM64 without SIMD prefilters, every candidate hit DFA → cache corruption → 1.7GB allocs, 1s+ per op on M2 Max.

Fix: prefilter used for fast rejection only (one Find call). Verification falls through to DFA.IsMatch (read-mostly when cache warm).

Test: TestConcurrentCaseInsensitivePrefilter — 8 goroutines × 100 iterations, match + no-match paths.

Reported by @tjbrains on Apple M2 Max (#137).

Test plan

• Full test suite passes
• Concurrent test passes (8 goroutines)
• Lint clean
• CI passes

[github-actions]

Benchmark Comparison

Comparing main → PR #145

Summary: geomean 118.4n 118.3n -0.04%

⚠️ Potential regressions detected:

Accelerate/memchr3-4       339.0n ± ∞ ¹   339.8n ± ∞ ¹  +0.24% (p=0.024 n=5)
geomean                               ³                +0.00%               ³
geomean                               ³                +0.00%               ³
geomean                         ³                +0.00%               ³
geomean                         ³                +0.00%               ³
AhoCorasickManyPatterns/coregex_25_patterns-4           72.05n ± ∞ ¹     73.54n ± ∞ ¹     +2.07% (p=0.016 n=5)
ASCIIOptimization/WithASCII-4                           9.428n ± ∞ ¹     9.677n ± ∞ ¹     +2.64% (p=0.024 n=5)
ASCIIOptimization_Issue79/medium_WithoutASCII-4         630.9n ± ∞ ¹     643.8n ± ∞ ¹     +2.04% (p=0.032 n=5)
LangArenaLogParser/suspicious-4                         1.188m ± ∞ ¹     1.202m ± ∞ ¹     +1.16% (p=0.032 n=5)
LangArenaLogParser/ips-4                                96.18µ ± ∞ ¹     97.33µ ± ∞ ¹     +1.20% (p=0.008 n=5)

Full results available in workflow artifacts. CI runners have ~10-20% variance.
For accurate benchmarks, run locally: ./scripts/bench.sh --compare

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
meta/ismatch.go	80.00%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!