feat: B-spline support (#203)

[ulises-jeremias]

poly/poly.v covers evaluation, quadratic, cubic, and companion matrix but no B-spline basis.

Task

Add poly/bspline.v with:

1. B-spline basis function evaluation
2. Knot insertion
3. Curve evaluation (de Boor's algorithm)
4. poly/bspline_test.v with known knot vectors

Reference

• Issue: Bspline polynomials #203