Every region-selection change re-downloaded every selected country's
.osm.pbf from scratch, even ones already fetched by a prior build —
painful for multi-GB countries. Extracts now persist in
data/geofabrik_cache/, keyed by country, and are only re-fetched when
Geofabrik's Last-Modified for that country actually moved on (checked
with a cheap HEAD) or the file isn't cached yet.