First slice of the NARR backfill client: url_for/1,
url_for_inventory/1, and snap_to_analysis_hour/1. Pure functions,
no network. Validates that NARR analyses only exist at 3-hourly
slots (00/03/06/09/12/15/18/21 UTC). The byte-range fetch and
cdo-driven point extraction land in follow-up commits per
docs/plans/2026-04-15-merra2-historical-backfill.md.
