feat(uc): add reassociation-stable scheduler model

[quangvdao]

Summary

The legacy UC process model installs the same Boolean sampler at every binary composition node. Consequently, a three-component system gives different effective scheduling weights to its leaves depending on whether it is parsed as (P ∥ Q) ∥ C, P ∥ (C ∥ Q), or Q ∥ (C ∥ P).

This PR adds the generic, probability-free foundation for fixing that problem:

• ScheduleMass records a strictly positive frontier weight and adds under composition.
• BinaryScheduler receives the masses of both subtrees instead of blindly flipping the same Boolean sampler.
• IsFlat states the key semantic law: every hierarchical three-way draw agrees with one parenthesization-free flat draw; IsCoherent derives the swap and reassociation laws needed by UC factorization.
• ScheduledOpenProcess preserves the existing binary process representation while carrying total frontier mass and using the mass-aware scheduler at composition nodes.
• A canary demonstrates that merely carrying masses is not enough: a deterministic mass-sensitive policy can still violate reassociation, so the IsFlat premise is substantive.

This is deliberately probability-neutral. The next PolyFun slice is #176, and the concrete downstream ProbComp scheduler and its exact denotational flatness proof are in Verified-zkEVM/VCVio#572.

Validation

• ./scripts/validate.sh --lint --test --axioms
• zero sorry/axiom debt across the PolyFun namespace