parse_inventory walks the wgrib1-format .inv text and returns a
{var, level} -> {byte_offset, length} index, computing each record's
length from the next record's offset (or file_size for the last one).
fetch_inventory does a GET on the .inv URL plus a HEAD on the .grb URL
to learn its size, then delegates to parse_inventory.
Both stubbable via Req.Test (config/test.exs adds narr_req_options
matching the existing era5_req_options / giro_req_options pattern).
Spike fixture at test/fixtures/narr/narr-a_221_20100615_1200_000.inv
backs the parser test — verifies the ("TMP", "2 m above gnd") and
("WVVFLX", "atmos col") records line up with the real bytes.
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.
