Robust invariance

Role. Guarantee. Graph node robust.

Compare ∃π ∀t: X_t^π ∈ K with ∀d∈D ∀t: X_t^{π,d} ∈ K. They support different deployment conclusions.

— Frames draft §3.2

Purple edge: strengthens ∃ control to ∀ disturbance. Lean (robust_strengthens_viable, viable_not_robust): a robust control is viable when the nominal disturbance is in the class, and not conversely. Red edge with correction: if correction inputs sit in the disturbance class and the target lies outside K, both cannot hold (robustCorrectionIncompat). Exempting corrections from the class is the boundary where both can hold.