test(bench): add ScaleN scalability suite and fix CI filter

[sentomk]

Summary

Add a variant dispatch scalability benchmark suite (ScaleN: 4/8/16/32 alternatives) and fix the CI benchmark filter that caused benchmarks to produce empty JSON.

Changes

• Fixed --benchmark_filter regex: removed $ anchors that prevented matching (Google Benchmark appends /min_time:.../repeats:... suffixes to names).
• Added bench_scale.cpp: measures Patternia vs std::visit vs switch(index) across 4, 8, 16, and 32 variant alternatives using a realistic event-type hierarchy.
• CI now builds and runs both ptn_bench and ptn_bench_scale, producing two separate JSON result files and two charts.
• Filtered out low-value literal-match scenarios from the main chart; ScaleN gets its own dedicated chart.
• Dropped LiteralMatch* from CI filter (pure int literal matching is switch's home turf and dilutes the comparison).

Testing

• Verified locally: both binaries compile, ScaleN produces 12 benchmark points (3 implementations x 4 scales).
• CI YAML validated.
• clang-format clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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