Add prior-corrected artifact aggregation

[FlorianPfaff]

Summary

• add prior-corrected mean-score artifact aggregation modes and aliases
• apply unlabeled per-participant class-prior correction in regular and nested-weight ensemble paths
• expose the modes in the artifact ensemble workflow and add regression coverage

Validation

• python -m py_compile src\\pymegdec\\stimulus_artifact_ensemble.py tests\\test_stimulus_artifact_ensemble.py
• git diff --check

Tests were not run per request.