Stoney stress & substrate warping

Compute the equibiaxial film stress from a measured substrate radius of curvature, or predict the resulting wafer bow for a target stress. 12 substrate materials.

Stoney equation (1909) implementation with 12 substrate mechanical models (Si, glass, polymers). Two directions: σ → R and R → σ. Includes sag calculation and a thin-film-limit validity check.