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.
Graham McIntire
081cd47bb5