docs: update ROADMAP.md to reflect v0.12.6

[kolkov]

Update ROADMAP.md version references from v0.12.5 to v0.12.6. Add v0.12.6 to release history and completed milestones.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[github-actions]

Benchmark Comparison

Comparing main → PR #130

Summary: geomean 121.7n 121.8n +0.13%

⚠️ Potential regressions detected:

LazyDFASimpleLiteral-4     59.96n ± ∞ ¹   60.41n ± ∞ ¹  +0.75% (p=0.016 n=5)
geomean                    121.7n         121.8n        +0.13%
geomean                               ³                +0.00%               ³
geomean                               ³                +0.00%               ³
geomean                         ³                +0.00%               ³
geomean                         ³                +0.00%               ³
AhoCorasickManyPatterns/coregex_10_patterns-4           58.16n ± ∞ ¹    60.60n ± ∞ ¹    +4.20% (p=0.016 n=5)
AhoCorasickManyPatterns/coregex_50_patterns-4           86.52n ± ∞ ¹    89.94n ± ∞ ¹    +3.95% (p=0.008 n=5)
ASCIIOptimization_Issue79/long_WithoutASCII-4           2.673µ ± ∞ ¹    2.691µ ± ∞ ¹    +0.67% (p=0.008 n=5)
Composite_Coregex/AlphaDigitAlpha-4                     60.03n ± ∞ ¹    61.79n ± ∞ ¹    +2.93% (p=0.032 n=5)

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