Vendor scorecard#183's base-side grid record and apply known gaps in the replay gate #636

Follow-up to #634 and #635 (UK replay gate policy d1344-replay-regressions-v1).

policyengine-scorecard#183 merged as 71cad306. Every replay receipt now carries:

• the comparison base's own grid record, diff.base_events.<event>, with these fields: staging and run manifest digests, supported years, full_supported_grid_complete, and the inert, missing and unsupported measure-years;
• the candidate's missing_measure_years and unsupported_measure_years lists.

policyengine.py's vendored replay_receipt.v1.json and its mirrors are closed, so every new receipt is refused until they are refreshed.

Work:

• Vendor the schema from scorecard main at 71cad306. Extend the mirrors, including the tightened measure-year lists: a non-empty measure_key and unique items.
• Implement the decision record's exact known-gap test. A candidate measure-year that is inert, or missing and supported, is a known gap only if the base's record for the same event lists it as inert, or missing and supported.