ci: switch to LLVM source-based coverage & expand benchmark scenarios

[sentomk]

Summary

Switch CI coverage from gcc gcov to LLVM source-based coverage, and expand benchmark scenario coverage from 4 to 10 scenarios.

Changes

• Coverage: --coverage (gcc gcov) -> -fprofile-instr-generate -fcoverage-mapping (Clang 18 source-based), using llvm-profdata-18 merge + llvm-cov-18 export instead of lcov. IIFE/macro expansions are now correctly attributed.
• Benchmark filter: 4 -> 10 scenarios, adding VariantAltHot, VariantMixedGuarded, VariantAltHotGuarded, PacketMixedHeavyBind, LiteralMatch, LiteralMatchRDense.
• Chart adaptation: figure height and axes area dynamically scale with scenario count; y-axis label font shrinks for >6 scenarios to prevent overlap.

Testing

CI fully passing: coverage job (clang-18 LLVM source-based) pass, build-and-test pass, full matrix compilation pass. Benchmark job conditionally runs only on merge to main (if: github.event_name == 'push' && github.ref == 'refs/heads/main').

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!