fix: Store#save writes non-default language posts into lang subdirectory

default_path_for now routes posts whose lang differs from default_lang
into content_dir/<lang>/<slug>.md, matching the documented multi-language
layout and preventing same-slug posts in different languages from
overwriting each other.

test/store_test.rb

@@ -55,4 +55,19 @@ class StoreTest < Minitest::Test
     post = Volumen::Post.new(metadata: { "slug" => "../../etc/passwd" }, body: "x")
     assert_raises(ArgumentError, "slug escapes content directory") { store.save(post) }
   end
+
+  def test_save_uses_lang_subdirectory_for_non_default_language
+    store = Volumen::Store.new(@dir, default_lang: "en")
+    post = Volumen::Post.new(metadata: { "slug" => "bonjour", "lang" => "fr" }, body: "x")
+    store.save(post)
+    assert File.exist?(File.join(@dir, "fr", "bonjour.md"))
+  end
+
+  def test_save_uses_root_for_default_language
+    store = Volumen::Store.new(@dir, default_lang: "en")
+    post = Volumen::Post.new(metadata: { "slug" => "hello-new", "lang" => "en" }, body: "x")
+    store.save(post)
+    assert File.exist?(File.join(@dir, "hello-new.md"))
+    refute File.exist?(File.join(@dir, "en", "hello-new.md"))
+  end
 end