Cache parsed GPX/FIT GeoJSON to fix large POI-export loads

A several-thousand-waypoint park4night area export re-parses the
whole file with REXML on every toggle — ~2.4s for a 5k-waypoint/3.6MB
sample, so tens of seconds for a 50k-waypoint one, likely the actual
cause behind a "Could not load" report on a 41.5MB export. MapLibre
already clusters the waypoint layer client-side, so the bottleneck was
never rendering — cache the GeoJSON conversion alongside the source
file (invalidated by mtime) instead of a bigger bbox/tiling rework.

test/test_gpx_store.rb

@@ -67,4 +67,59 @@ class GpxStoreTest < Minitest::Test
     assert_equal "archive/ride-1.gpx", new_path
     assert_empty @store.tree["children"].reject { |c| c["name"] == "archive" }
   end
+
+  def test_geojson_is_cached_alongside_the_source_file
+    path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+    cache_path = @store.resolve_file(path) + ".geojson"
+    refute File.file?(cache_path)
+
+    first = @store.geojson(path)
+    assert File.file?(cache_path)
+
+    # Corrupt the cache directly to prove the second call reused it instead
+    # of re-parsing the (still-valid) source file.
+    File.write(cache_path, first.sub("waypoint", "cached-waypoint"))
+    assert_includes @store.geojson(path), "cached-waypoint"
+  end
+
+  def test_geojson_cache_is_invalidated_when_the_file_is_resaved
+    path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+    @store.geojson(path)
+
+    File.write(@store.resolve_file(path), sample_gpx.sub("Waypoint", "Renamed"))
+    FileUtils.touch(@store.resolve_file(path), mtime: Time.now + 1)
+
+    assert_includes @store.geojson(path), "Renamed"
+  end
+
+  def test_delete_file_removes_the_geojson_cache_too
+    path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+    @store.geojson(path)
+    cache_path = @store.resolve_file(path) + ".geojson"
+
+    @store.delete_file(path)
+
+    refute File.file?(cache_path)
+  end
+
+  def test_move_relocates_the_geojson_cache_too
+    path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+    @store.geojson(path)
+
+    new_path = @store.move(from: path, to_folder: "archive")
+
+    assert File.file?(@store.resolve_file(new_path) + ".geojson")
+  end
+
+  def test_geojson_raises_enoent_for_a_missing_file
+    assert_raises(Errno::ENOENT) { @store.geojson("nope.gpx") }
+  end
+
+  private
+
+  def sample_gpx
+    <<~GPX
+      <gpx><wpt lat="48.1" lon="17.1"><name>Waypoint</name></wpt></gpx>
+    GPX
+  end
 end