Stream Geofabrik extract downloads instead of buffering in memory

Net::HTTP's plain #get loads the whole response body before returning —
fine for the small JSON manifests elsewhere in this file, but a real risk
for a multi-GB country .osm.pbf (Germany, Russia, ...). Streams to disk
via read_body instead.

lib/maptool/pmtiles_builder.rb

@@ -262,16 +262,23 @@ module Maptool
     private_class_method :geofabrik_url
 
     # Real network implementation of the download_extract seam above —
-    # streams the extract to dest and returns its Last-Modified header
+    # streams the extract to dest (some of these are multi-GB — Net::HTTP's
+    # plain #get buffers the whole response body in memory first, which
+    # read_body's block form avoids) and returns its Last-Modified header
     # (free off the same GET, no separate HEAD needed) for the sidecar
     # meta / update-check below.
     def self.download_extract(url, dest)
       uri = URI(url)
-      response = Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https") { |http| http.get(uri) }
+      last_modified = nil
+      Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https") do |http|
+        http.request_get(uri) do |response|
           raise BuildError, "could not download #{url} (HTTP #{response.code})" unless response.is_a?(Net::HTTPSuccess)
 
-      File.binwrite(dest, response.body)
-      response["last-modified"]
+          last_modified = response["last-modified"]
+          File.open(dest, "wb") { |file| response.read_body { |chunk| file.write(chunk) } }
+        end
+      end
+      last_modified
     end
 
     # Sidecar recording which Geofabrik sources (country code, slug, and