Pass --download to Planetiler for its auxiliary OpenMapTiles sources

Real build failure once the tmp-filename fix got past the format-
detection error: "data/sources/lake_centerline.shp.zip does not exist.
Run with --download to fetch it". The OpenMapTiles profile needs a few
small non-OSM sources (water polygons, Natural Earth, lake
centerlines) alongside the .osm.pbf itself, and Planetiler only
fetches them when told to. They land under its own default
data/sources/, gitignored alongside the rest of data/ — fetched once,
reused by every later build.

lib/maptool/pmtiles_builder.rb

@@ -268,7 +268,15 @@ module Maptool
       # suffix made it fail with "Unsupported format tmp" instead of
       # actually tiling anything.
       tmp_path = File.join(data_dir, "offline.tmp.pmtiles")
-      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}")
+      # --download: the OpenMapTiles profile also needs a few small auxiliary
+      # sources (water polygons, Natural Earth, lake centerlines) alongside
+      # the OSM extract itself — without this flag Planetiler expects them
+      # to already exist on disk and fails instead of fetching them. They
+      # land under Planetiler's own default "data/sources/", relative to
+      # this process's working directory (the app root, so in practice
+      # that's data_dir's own "sources" subdirectory) — fetched once,
+      # reused by every later build.
+      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}", "--download")
       File.rename(tmp_path, File.join(data_dir, "offline.pmtiles"))
       File.write(offline_meta_path(data_dir), JSON.generate(sources: sources.map { |s| s.slice("code", "slug", "last_modified") }))
     ensure