Add corner‑case tests for core simplify

  - Target: [iOverlay/src/core/simplify.rs](https://github.com/iShape-Rust/iOverlay/blob/main/iOverlay/src/core/simplify.rs)
  - Add tests for minimal inputs (0–2 points) and near‑collinear paths
  - Assert invariants (no self‑intersections introduced)