mixle.fault module

Compatibility shim: mixle.fault moved to mixle.system.fault.

The degradation policy (with_fallback, abstain_on_timeout, route_past, DegradedResult) is now part of one cohesive package, mixle.system, instead of six cross-importing top-level modules. Existing from mixle.fault import X imports keep working (with a DeprecationWarning); new code should import from mixle.system (or mixle.system.fault) directly.