feat: add fediverse_creator handle for Mastodon attribution

Expose a per-post fediverse_creator frontmatter field plus a site-wide
default in config.toml ([site].fediverse_creator). The handle is returned
in the post and site payloads, rendered as <dc:creator> in the RSS feed
and as authors[].name in the JSON feed, so a front-end can drop it into
<meta name="fediverse:creator"> on rendered pages. Per-post value
overrides the site default. Validated against a simple @user@host regex.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

test/post_test.rb

@@ -77,4 +77,61 @@ class PostTest < Minitest::Test
     post = Volumen::Post.new(metadata: {})
     refute_predicate post, :scheduled?
   end
+
+  def test_fediverse_creator_returns_metadata_value
+    post = Volumen::Post.new(
+      metadata: { "slug" => "x", "fediverse_creator" => "@user@example.social" }
+    )
+    assert_equal "@user@example.social", post.fediverse_creator
+  end
+
+  def test_fediverse_creator_is_nil_when_missing
+    post = Volumen::Post.new(metadata: { "slug" => "x" })
+    assert_nil post.fediverse_creator
+  end
+
+  def test_fediverse_creator_is_nil_when_blank
+    post = Volumen::Post.new(metadata: { "slug" => "x", "fediverse_creator" => "   " })
+    assert_nil post.fediverse_creator
+  end
+
+  def test_fediverse_creator_validation_accepts_well_formed_handles
+    post = Volumen::Post.new(
+      metadata: { "slug" => "x", "fediverse_creator" => "@ada@lovelace.org" }
+    )
+    assert_predicate post, :valid_fediverse_creator?
+  end
+
+  def test_fediverse_creator_validation_rejects_malformed_handles
+    [
+      "ada@example.com",
+      "@only-one-at",
+      "@user@example.com extra",
+      "@user@",
+      "@@example.com"
+    ].each do |value|
+      post = Volumen::Post.new(metadata: { "slug" => "x", "fediverse_creator" => value })
+      refute_predicate post, :valid_fediverse_creator?, "expected #{value.inspect} to be invalid"
+    end
+  end
+
+  def test_fediverse_creator_validation_accepts_padded_value
+    post = Volumen::Post.new(
+      metadata: { "slug" => "x", "fediverse_creator" => "  @user@example.com  " }
+    )
+    assert_predicate post, :valid_fediverse_creator?
+  end
+
+  def test_summary_includes_fediverse_creator
+    post = Volumen::Post.new(
+      metadata: { "slug" => "x", "fediverse_creator" => "@user@example.social" },
+      body: "body"
+    )
+    assert_equal "@user@example.social", post.summary["fediverse_creator"]
+  end
+
+  def test_summary_omits_fediverse_creator_when_absent
+    post = Volumen::Post.new(metadata: { "slug" => "x" }, body: "body")
+    assert_nil post.summary["fediverse_creator"]
+  end
 end