Ported for Lean Pool: imports relocated and code adapted to Lean/Mathlib v4.34.0.

Semijet calculus

This file contains calculus lemmas for second-order superjets and subjets: addition, convexity, shifts, Hessian monotonicity, fixed-gradient Hessian slice closedness, and negation duality. The development lives in the Calculus/ submodules; this file re-exports all of them.