Surface normal and the two principal curvature directions.

Mirrors tbp/monty/geometry.py::SurfacePoseVectors.

value is a 3×3 matrix in row-major order, in [z (surface normal), x (principal curvature 1), y (principal curvature 2)] order. It is meant to be a proper rotation (determinant 1, orthonormal rows); like the Python class this module does not validate that, because Monty itself constructs degenerate instances when the curvature estimate is flat.