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.

test/test_pmtiles_builder.rb

@@ -102,8 +102,12 @@ class PmtilesBuilderTest < Minitest::Test
   # download_extract stub: writes a marker file (same "fake" content the
   # pmtiles/osmium/java fixtures use) instead of hitting Geofabrik, and
   # returns a fixed Last-Modified so sidecar-meta assertions are stable.
-  def fake_download(last_modified: "Wed, 10 Sep 2026 08:00:01 GMT")
-    ->(_url, dest) {
+  # `calls` (an Array, shared across builds if the caller wants it) records
+  # every URL actually downloaded, for asserting the per-country cache
+  # skipped (or didn't skip) a re-download.
+  def fake_download(last_modified: "Wed, 10 Sep 2026 08:00:01 GMT", calls: nil)
+    ->(url, dest) {
+      calls << url if calls
       File.write(dest, "fake")
       last_modified
     }
@@ -189,6 +193,73 @@ class PmtilesBuilderTest < Minitest::Test
     assert_equal %w[SK AT], codes
   end
 
+  # The actual bug report this covers: hitting "Apply" after tweaking the
+  # region selection used to re-download every selected country's .osm.pbf
+  # from scratch, even ones already fetched by an earlier build.
+  def test_build_offline_skips_redownloading_an_unchanged_cached_country
+    select_regions("SK")
+    calls = []
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download(calls: calls)).join
+    assert_equal 1, calls.size
+
+    Maptool::PmtilesBuilder.build_offline(
+      data_dir: @data_dir,
+      download_extract: fake_download(calls: calls),
+      fetch_last_modified: ->(_url) { "Wed, 10 Sep 2026 08:00:01 GMT" },
+    ).join
+
+    assert_equal 1, calls.size
+    assert_equal "done", Maptool::PmtilesBuilder.offline_status[:state]
+  end
+
+  def test_build_offline_redownloads_a_country_whose_extract_changed
+    select_regions("SK")
+    calls = []
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download(calls: calls)).join
+
+    Maptool::PmtilesBuilder.build_offline(
+      data_dir: @data_dir,
+      download_extract: fake_download(calls: calls, last_modified: "Thu, 11 Sep 2026 08:00:01 GMT"),
+      fetch_last_modified: ->(_url) { "Thu, 11 Sep 2026 08:00:01 GMT" },
+    ).join
+
+    assert_equal 2, calls.size
+  end
+
+  # A cache-freshness check that itself fails (Geofabrik unreachable) should
+  # fall back to re-downloading rather than aborting the build.
+  def test_build_offline_redownloads_when_the_cache_check_fails
+    select_regions("SK")
+    calls = []
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download(calls: calls)).join
+
+    fetch_last_modified = ->(_url) { raise Maptool::PmtilesBuilder::BuildError, "could not reach geofabrik" }
+    Maptool::PmtilesBuilder.build_offline(
+      data_dir: @data_dir,
+      download_extract: fake_download(calls: calls),
+      fetch_last_modified: fetch_last_modified,
+    ).join
+
+    assert_equal 2, calls.size
+    assert_equal "done", Maptool::PmtilesBuilder.offline_status[:state]
+  end
+
+  def test_build_offline_only_downloads_a_newly_added_country
+    select_regions("SK")
+    calls = []
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download(calls: calls)).join
+
+    select_regions("SK", "AT")
+    Maptool::PmtilesBuilder.build_offline(
+      data_dir: @data_dir,
+      download_extract: fake_download(calls: calls),
+      fetch_last_modified: ->(_url) { "Wed, 10 Sep 2026 08:00:01 GMT" },
+    ).join
+
+    assert_equal 1, calls.count { |url| url.include?("slovakia") }
+    assert_equal 1, calls.count { |url| url.include?("austria") }
+  end
+
   def test_offline_meta_is_nil_before_any_build
     assert_nil Maptool::PmtilesBuilder.offline_meta(@data_dir)
   end