Stage 3 should verify the Stage 2 calibration package contract before fitting starts.

Scope:

• read calibration_package_contract.json from the Stage 3 input bundle;
• reject missing or mismatched package/contract identity for normal pipeline runs;
• record package and contract checksums in fit manifests;
• keep manual no-contract package fitting as an explicit warning-only fallback;
• cover the contract handoff with focused unit tests and Stage 3 AI-facing docs.