chore: release v1.2.0

- Update CHANGELOG.md for v1.2.0 (convert [Unreleased] to [1.2.0] - 2026-06-10)
- Add benchmark results (benchmarks/results/v1.2.0.txt)
- Update BENCHMARKS.md with v1.2.0 vs v1.1.0 comparison tables
- Security clearance: CLEARED 2026-06-10 (all CRITICAL/HIGH/MEDIUM findings from 2026-06-09 audit confirmed remediated)

Author: FlavioCFOliveira