Prototype: extract offline hiking trails from the Geofabrik PBF we already have

No custom Planetiler profile needed after all — Maptool::HikingTrails
runs osmium tags-filter r/route=hiking against the same .osm.pbf
PmtilesBuilder already downloads per country for offline.pmtiles, then
osmium cat -f opl (locations_on_ways=true) to get member-way geometry
back as plain text, no separate node-id lookup table to build.

No general OSM tool stitches a route relation into one line (routes
are a semantic grouping, not a geometry type) — parse_opl does that
walk itself, same as the Overpass-based click handler already does
client-side. Kept pure/testable apart from the two osmium subprocess
calls, mirroring OverpassPois.fetch vs .to_geojson.

rake hiking:trails[pbf,out] to try it by hand against a real cached
extract. Not wired into the actual offline build or rendering yet —
still just a prototype, per TODO.adoc.

Rakefile

@@ -24,6 +24,22 @@ namespace :overpass do
   end
 end
 
+namespace :hiking do
+  desc "Extract route=hiking relations (name/colour/network/operator, full geometry) from a local " \
+       ".osm.pbf (e.g. one of PmtilesBuilder's own data/geofabrik_cache/*.osm.pbf) and write GeoJSON. " \
+       "Prototype — see lib/maptool/hiking_trails.rb. Usage: rake hiking:trails[input.osm.pbf,out.geojson]"
+  task :trails, %i[pbf out] do |_t, args|
+    %i[pbf out].each { |key| abort("usage: rake hiking:trails[input.osm.pbf,out.geojson]") unless args[key] }
+
+    require_relative "lib/maptool/hiking_trails"
+
+    puts "Extracting hiking routes from #{args[:pbf]}..."
+    geojson = Maptool::HikingTrails.extract(args[:pbf])
+    File.write(args[:out], JSON.generate(geojson))
+    puts "Wrote #{geojson["features"].size} route(s) to #{args[:out]}"
+  end
+end
+
 namespace :gpx do
   desc "Backfill properties.id on Note-category features saved before edit/delete (and its id targeting) existed"
   task :backfill_note_ids do