feat: implement Longest() for stdlib compatibility (Fixes #26)

[kolkov]

Summary

Implements proper Longest() method that switches from leftmost-first (Perl) to leftmost-longest (POSIX) matching semantics.

Before: Longest() was a no-op stub with misleading documentation claiming coregex used leftmost-longest by default.

After: Longest() actually works, matching Go stdlib regexp.Regexp.Longest() behavior.

Changes

• Add longest bool flag to Regex struct
• Implement SetLongest() in meta.Engine and nfa.PikeVM
• Modify PikeVM search functions to support longest semantics:
  • searchUnanchoredAt()
  • searchUnanchoredWithCapturesAt()
  • searchAt()
  • searchAtWithCaptures()
• Fix misleading documentation
• Add comprehensive tests

Behavior

Pattern	Input	Default (leftmost-first)	After Longest() (leftmost-longest)
(a|ab)	"ab"	"a"	"ab"
(#|#!)	"#!a"	"#"	"#!"
(cat|catalog)	"catalog"	"cat"	"catalog"

Performance

No regression in default mode!

Package	Change
Lazy DFA	-9.37% (faster)
OnePass	-15.87% (faster)
Meta	+5.58% (compile time only)
Prefilter	-4.48% (faster)

Test plan

• go test ./... passes
• TestLongest - verifies behavior change after Longest()
• TestLongestMatchesStdlib - verifies match with Go stdlib
• Benchmark comparison shows no regression

Fixes #26

[github-actions]

Benchmark Comparison

Comparing main → PR #27

Summary: geomean 404.8n 391.1n -3.38%

⚠️ Potential regressions detected:

Accelerate/memchr3-4       338.8n ± ∞ ¹   340.9n ± ∞ ¹   +0.62% (p=0.024 n=5)
LazyDFASimpleLiteral-4     697.7n ± ∞ ¹   701.4n ± ∞ ¹   +0.53% (p=0.008 n=5)
geomean                               ³                +0.00%               ³
geomean                               ³                +0.00%               ³
geomean                         ³                +0.00%               ³
geomean                         ³                +0.00%               ³
Compile/\d+-4                                           1.793µ ± ∞ ¹    1.907µ ± ∞ ¹   +6.36% (p=0.008 n=5)
Find/hello-4                                            705.5n ± ∞ ¹    722.9n ± ∞ ¹   +2.47% (p=0.008 n=5)
Find/\d+-4                                              661.0n ± ∞ ¹    667.9n ± ∞ ¹   +1.04% (p=0.008 n=5)
ReverseSuffix_IsMatch_Comparison/coregex_txt_32KB-4     582.5n ± ∞ ¹    658.9n ± ∞ ¹  +13.12% (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