Rust-side writer for the f00 ProfilesFile. Replaces the Elixir ETF
format (:erlang.term_to_binary) with MessagePack + gzip so Rust can
produce files without porting the Erlang serializer.
Wire layout: top-level map with 'v' (format version), 'valid_time'
(ISO string), and 'cells' (array of {lat, lon, profile}). Profile
values are an rmpv::Value so the writer stays agnostic about what
fields the f00 pipeline decides to include — native-duct metrics,
NEXRAD reflectivity, commercial-link degradation, and the sounding
profile list all serialize through the same path.
All keys are strings — MessagePack has no atom type. Elixir's reader
(follow-up commit) converts known keys to atoms via a whitelist.
Atomic writes via rename(2) mirror ScoresFile/ProfilesFile pattern:
write to tmp sibling, rename into place on the shared NFS mount.
4 unit tests cover round-trip, atomic rename, path layout, and
lat/lon snap.
Graham McIntire
1307b1d8ac