Adds the missing link between the GIRO ionosonde data we just started
ingesting and the VHF/UHF band scoring:
* `Microwaveprop.Propagation.SporadicE` — ITU-R P.534-6 / Davies 1990
thin-layer Es MUF (sec(i) · foEs with h=110 km), plus an `es_score/3`
that maps (foEs, target band, hop distance) into a 0-100 single-hop
propagation likelihood. Calibrated against literature: 50 MHz Es at
2000 km needs foEs ≳ 5.5 MHz (routine summer); 144 MHz needs
foEs ≳ 15.8 MHz (rare Jun/Jul peaks only); 440 MHz Es does not occur
at physical foEs values. Multi-hop Es and Es-scatter are separate
factors and explicitly out of scope.
* `Ionosphere.nearest_foes/3` — given a lat/lon, returns the latest
observation from the nearest polled GIRO station (Millstone Hill or
Alpena for now), with a configurable staleness cutoff (default 2h).
Returns `{:error, :stale}` or `{:error, :no_data}` so callers can
choose whether to apply the Es factor at all.
Neither is wired into the grid scorer yet — that's a separate commit
so the integration can be reviewed on its own.
Graham McIntire
f632ea83bd