mixle.scorecard module

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

SystemScorecard and its evaluation/regression helpers are now part of one cohesive package, mixle.system, instead of six cross-importing top-level modules. Existing from mixle.scorecard import X imports keep working (with a DeprecationWarning); new code should import from mixle.system (or mixle.system.scorecard) directly. Not to be confused with mixle.task.scorecard.Scorecard, a different, narrower comparison (one student solution vs its teacher on a task) that did not move.