mixle.spend module

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

Spend and its siblings (System, degradation modes, the scorecard, the meta-improvement allocator, and the registry) are now one cohesive package, mixle.system, instead of six cross-importing top-level modules. Existing from mixle.spend import X imports keep working (with a DeprecationWarning); new code should import from mixle.system (or mixle.system.spend) directly.