Improving CI and Benchmarking for Manifold

This project improves Manifold’s CI and benchmarking by adding missing determinism, sanitizer, and performance regression checks. I will implement a required cross-platform determinism check (Linux/macOS/Windows), add a dedicated ASan+UBSan CI lane for a curated `manifold_test` subset, and introduce PR benchmark regression checks (`base` vs `head`) with non-blocking warnings. I will also add a scheduled benchmark pipeline that publishes versioned history and define a noise-control policy so results stay reliable. The final deliverable includes an interactive benchmark dashboard that visualizes trends and regressions over time using the published benchmark data.