Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Promotion is a gate, not a mood: write promote_gated(name, version, candidate_acc). If a production incumbent exists and the candidate's accuracy is LOWER, print f"REFUSED: candidate acc {c} < incumbent {i}" and return False, leaving the registry untouched. Otherwise archive the incumbent, register the candidate as production, print f"promoted {name} v{version} (acc {a})", return True. Try a worse candidate (v4 @ 0.82 — refused) then a better one (v5 @ 0.89 — promoted), then print every entry as f"{n} v{v}: {stage} acc={acc}" (sorted).