Accept NOAA-omitted freezing fields in RRFS/REFS decode validation

[aldenks]

NOAA RRFS/REFS indexes have omitted freezing-rain and frozen-precipitation messages since the 2026-10-15 12Z cycle. Updates ingest the other fields, but post-commit decode validation fails because the omitted fields read as all NaN.

Add explicit, documented source-missing variable lists for RRFS 18-hour, 84-hour, sub-hourly and ensemble, plus REFS hourly and 3-hourly. These cover FRZR/FROZR variables, including deterministic frozen_precipitation_surface and every REFS freezing-rain probability window/threshold; categorical freezing rain remains strictly checked. The decode validator accepts all-NaN samples caused by missing refs only for these names. Present all-NaN refs and decode errors still fail, so validation resumes automatically when NOAA restores a field. Skipped variable names appear in validation output. Future absences of these named fields remain allowed until the lists are removed. Existing all-NaN exceptions continue requiring refs. All other variables, freshness and file completeness keep their existing checks.

Ingestion is unchanged: restored messages in newly ingested files are retained normally. Already-ingested files are not automatically revisited.

Validation: local regression tests cover missing fields, all-NaN fields, restored values, decode errors, unrelated all-NaN variables and unknown exception names; dataset tests pin the exceptions to FRZR/FROZR. All 189 tests in the targeted validation, virtual-region, RRFS/REFS and offline decode suites pass, along with the six final focused omission cases. Format, lint and type checks pass. Final-head CI is green: amd64/arm64 NOAA and other tests, Kubernetes admission, Code Quality and CodeQL. Independent review is complete with no blocking findings.

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