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feat(api): GET /experiments/runs + POST /experiments/diff (Track 5)

feat(api): POST /explain/eeg + /explain/mri — full-stack Track-1 coverage

feat(api): POST /explain/bbb — natural-language rationale endpoint

feat(api+frontend): MLflow provenance badge in decision card

feat(api): drift z-score in /predict/bbb response

feat(api): POST /pipeline/mri/diagnostics — pre/post ComBat KPIs + long-format rows

feat(api): expose calibration bin in /predict/bbb response

feat(api): POST /predict/bbb with prediction, uncertainty, SHAP top-k

feat(api): POST /pipeline/{bbb,eeg,mri} dispatch routes