Path calculator now samples nine HRRR grid cells evenly along the
source→destination line (was three: Source/Midpoint/Destination) so
mid-path ducts that endpoint-only queries missed show up in the duct
count. The UI reports "Duct at N / 9 HRRR points" instead of a binary
flag, and the table lists every sample with its percent-of-path label.
Independent RAOB signal: uses the new `Weather.nearest_sounding_to/3`
to find the closest sounding within 300 km / ±3 h of the midpoint. If
the sounding's `ducting_detected` is true we surface a "RAOB <code>
confirms duct" badge — HRRR under-reads thin surface ducts that
soundings resolve, so the two signals together are the right
cross-check for live duct prediction.
Distance formatting: all km values on the path results (top Distance
summary, sounding "km from midpoint") now render as
`123 km (76 mi)`; under 10 km we keep one decimal so near-LOS paths
don't round to zero.