Flat LiRPA Engine

Executable graph engine for interval propagation, affine forms, and CROWN-style transfer rules.

• Engine.Base: flat vectors, boxes, parameter stores, and shared tensor helpers.
• Engine.IBP: interval bound propagation.
• Engine.Derivatives: first- and second-derivative interval passes.
• Engine.Affine: affine-form propagation.
• Engine.CROWN: forward CROWN/DeepPoly bounds.
• Engine.BackwardObjective: objective-dependent backward CROWN.