Cache per-country Geofabrik extracts so Apply doesn't redownload everything

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.

docs/development.adoc

@@ -99,10 +99,16 @@ There's no standalone per-country file to individually delete —
 the *whole current selection*. "Remove a country" and "refresh a
 country" are both just "change the selection, then Apply" (unticking a
 country and applying rebuilds without it; nothing to separately
-delete). Every Apply re-extracts the entire selection from scratch, not
-just what changed — fine given extraction is fast (byte-range requests
-against remote archives, not per-tile fetches), but worth revisiting if
-someone selects half of Europe at once and finds it slow.
+delete). terrain.pmtiles re-extracts its selection from scratch on
+every Apply — fine given that extraction is byte-range requests
+against a remote archive, not full downloads. offline.pmtiles, by
+contrast, downloads a whole country's `.osm.pbf` from Geofabrik (can be
+multi-GB), so it keeps a per-country cache under
+`data/geofabrik_cache/` instead of re-fetching on every Apply — a
+country already cached is only re-downloaded if Geofabrik's own
+`Last-Modified` for it has moved on since (checked with a cheap `HEAD`,
+see `PmtilesBuilder.cached_extract`), so growing or shrinking the
+selection only touches whatever's actually new to it.
 
 This wraps exactly the manual workflows documented above
 (`lib/maptool/pmtiles_builder.rb`). offline.pmtiles downloads each