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.

lib/volumen/store.rb

@@ -97,7 +97,9 @@ module Volumen
     end
 
     def default_path_for(post)
-      target = File.join(@content_dir, "#{post.slug}.md")
+      dir = @content_dir
+      dir = File.join(dir, post.lang) if post.lang && post.lang != @default_lang
+      target = File.join(dir, "#{post.slug}.md")
       unless File.expand_path(target).start_with?(File.expand_path(@content_dir) + File::SEPARATOR)
         raise ArgumentError, "slug escapes content directory"
       end

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