Enable SIMPLE until wall

[krystophny]

This issue mixed two approaches:

• Chartmap boundary at rho=1 (already supported operationally via coord_input chartmaps)
• CGAL ray/segment–mesh intersection (now tracked separately)

To keep one canonical spec for STL-based wall intersections (extended chartmap + rho_lcfs gate + explicit STL units), use:

• feat: wall_input STL intersection mode gated by rho_lcfs #308

GECKO owns STL generation + visualization:

• https://github.com/itpplasma/GECKO/issues/2

Closing to avoid multiple overlapping umbrella issues.