Multi-fidelity support: cheap surrogate then expensive model

[jc-macdonald]

Problem

All trials run at the same fidelity. In engineering DOE it's common to run a cheap surrogate (coarse mesh, simplified physics) first, then evaluate only promising designs with an expensive high-fidelity model.

Proposed

• Allow Phase to accept different Simulator instances (currently the Study-level world is used for all phases)
• A phase-level world override: Phase(name=..., grid=..., world=cheap_sim)
• Or a fidelity parameter that gets passed to the simulator's generate() via config

Design notes

This is a breaking change to Phase — needs careful API design. Consider whether the fidelity should be a phase property or a factor.