Extend Bao to compose MILP solutions across function calls

Develop and implement the proposed compositional extension that solves the mixed-integer linear program for boundary-capable functions separately and composes each callee's summary energy costs with the MILP solutions of its callers.

Background

Bao's global formulation currently assumes that functions are inlined into a single procedure. The paper outlines a compositional approach for functions whose worst-case energy exceeds the device budget: solve each callee independently, summarize entry-to-boundary, boundary-to-return, and boundary-free path costs, and use those summaries in callers. The implementation of this extension remains unresolved, and the authors note that compositional solving would preserve safety but not global optimality.