Create src/shells/api.jl defining shell-specific abstractions:
- AbstractShellTheory base type for shell theories
- ReissnerMindlin concrete theory (thick shells, includes shear, h/L > 1/20, 5 DOFs)
- KirchhoffLove concrete theory (thin shells, no shear, h/L < 1/20, 3 DOFs)
- ShellFormulation{Theory} parameterized formulation struct
Reissner-Mindlin has 5 DOFs per node (ux, uy, uz, θx, θy) with explicit
rotations. Kirchhoff-Love has 3 DOFs (ux, uy, uz) with rotations computed
from displacement gradients (normals remain perpendicular).
Part of systematic modular API architecture.
Jukka Aho