geo::crossings_between issued one DB round trip per consecutive GPS
sample pair (tens of thousands per multi-hour activity), even though
almost every segment stays inside a single county and produces no
crossing at all.
geo::crossings_for_track batches all segments of a track into a
single query via UNNEST ... WITH ORDINALITY. The per-segment geometry
logic (endpoint county lookup via the GiST index, boundary
intersection, ST_LineLocatePoint for the fractional position) is
unchanged, it now just runs once per activity instead of once per
segment.
