tweaks

test/config_test.rb

@@ -9,6 +9,7 @@ class ConfigTest < Minitest::Test
     assert_in_delta 463.0, config.maximum_diameter("eight_piece"), 0.001
     assert_in_delta 1.6, config["shell"]["wall_mm"], 0.001
     assert_in_delta 0.8, config["shell"]["minimum_feature_mm"], 0.001
+    assert_equal "solid", config["shell"]["interior"]
     assert_in_delta 10.0, config["stand"]["base_clearance_mm"], 0.001
   end
 
@@ -32,4 +33,8 @@ class ConfigTest < Minitest::Test
       MoonModel::Config.new("stand" => { "base_clearance_mm" => -0.1 })
     end
   end
+
+  def test_rejects_unknown_interior_mode
+    assert_raises(ArgumentError) { MoonModel::Config.new("shell" => { "interior" => "foam" }) }
+  end
 end