Introduce a minimal potential-response core

[bocowgill]

Summary

• Define a generic, assumption-free PotentialResponse type.
• Add pointwise selection and same-unit substitution.
• Add application, identity, and associativity laws.

Scope

This PR is purely functional. It introduces no probability or measurability structure, causal assumptions, estimands, identification results, estimators, statistical inference, or dependency on structural or graphical APIs.

Verification

• lake build
• lake lint
• lake exe mk_all --check

Closes #40.

[zixiaowang17]

Hi @bocowgill I have approved the building

[bocowgill]

Thanks—the build passed!

[bocowgill]

@rajarshi-mukherjee24: this is ready for review and CI passes. Could you take a look, or suggest an appropriate reviewer? It implements the revised scope of #40, including the documentation clarifications and substitution laws we discussed. Thanks!