Add zero-allocation scalar surface evaluator (nurbs_eval_derivs)

[mdmaas]

Adds nurbs_eval_derivs(patch, s, t) which evaluates a NURBS surface and its first partial derivatives at a single scalar point, returning (pos, dpds, dpdt) as three SVector{3}.

The existing API requires wrapping scalars in Vector wrappers and returns nested heap-allocated arrays, making it unsuitable for hot loops. The new path uses MMatrix/MVector scratch buffers sized by Val{degree} type parameters so all intermediates live on the stack — zero heap allocations, fully thread-safe.

Also fixes a minor allocation in derBasisFun!: the slice assignment dersv[jj, :, :] = ders is replaced with an explicit loop to avoid the temporary.

[weymouth]

Thanks! I think this closes #13. @HoBeZwe, is there a way to merge this? Anything not working?

[HoBeZwe]

Hi, and sorry for the late reply. I should have time to look at it soon.