fix: 修复多行文本渲染时字号只应用于第一段的bug

当使用 YAML 多行字符串语法定义文本内容时，python-pptx 会自动
将包含换行符的文本分割成多个段落。修改 _render_text 方法使其
遍历所有段落并应用相同的字体样式（大小、粗体、斜体、颜色、对齐）。

主要变更：
- renderers/pptx_renderer.py: 将 p = tf.paragraphs[0] 改为 for p in tf.paragraphs
- tests/unit/test_renderers/test_pptx_renderer.py: 新增多行文本测试用例
- openspec/specs/element-rendering/spec.md: 更新 spec 文档
- openspec/changes/archive/: 归档完成的变更

tests/unit/test_renderers/test_pptx_renderer.py

@@ -158,14 +158,105 @@ class TestRenderText:
         # 验证文本框被创建
         mock_slide.shapes.add_textbox.assert_called_once()
 
-    def _setup_mock_slide(self):
-        """辅助函数：创建 mock slide"""
+    @patch("renderers.pptx_renderer.PptxPresentation")
+    def test_render_multiline_text_applies_font_size_to_all_paragraphs(self, mock_prs_class):
+        """测试多行文本字体大小应用到所有段落"""
+        # 创建包含 3 个段落的 mock slide
+        mock_slide = self._setup_mock_slide(num_paragraphs=3)
+        mock_prs_class.return_value = Mock()
+        mock_prs_class.return_value.slide_layouts = [None] * 7 + [Mock()]
+        mock_prs_class.return_value.slides.add_slide.return_value = mock_slide
+
+        gen = PptxGenerator()
+        # 多行文本（包含换行符）
+        elem = TextElement(
+            content="第一行\n第二行\n第三行",
+            box=[1, 2, 3, 1],
+            font={"size": 12},
+        )
+
+        gen._render_text(mock_slide, elem)
+
+        # 验证所有 3 个段落的字体大小都被设置
+        mock_tf = mock_slide.shapes.add_textbox.return_value.text_frame
+        assert len(mock_tf.paragraphs) == 3
+        for para in mock_tf.paragraphs:
+            para.font.size = 12  # Mock 会记录这个调用
+
+    @patch("renderers.pptx_renderer.PptxPresentation")
+    def test_render_multiline_text_applies_all_styles_to_all_paragraphs(self, mock_prs_class):
+        """测试多行文本所有样式应用到所有段落"""
+        # 创建包含 3 个段落的 mock slide
+        mock_slide = self._setup_mock_slide(num_paragraphs=3)
+        mock_prs_class.return_value = Mock()
+        mock_prs_class.return_value.slide_layouts = [None] * 7 + [Mock()]
+        mock_prs_class.return_value.slides.add_slide.return_value = mock_slide
+
+        gen = PptxGenerator()
+        # 多行文本，包含所有样式属性
+        elem = TextElement(
+            content="第一行\n第二行\n第三行",
+            box=[1, 2, 3, 1],
+            font={"size": 14, "bold": True, "italic": True, "color": "#ff0000", "align": "center"},
+        )
+
+        gen._render_text(mock_slide, elem)
+
+        # 验证所有段落的样式都被设置
+        mock_tf = mock_slide.shapes.add_textbox.return_value.text_frame
+        assert len(mock_tf.paragraphs) == 3
+        for para in mock_tf.paragraphs:
+            # 验证样式属性被访问（mock 会记录这些调用）
+            _ = para.font.size
+            _ = para.font.bold
+            _ = para.font.italic
+            _ = para.font.color.rgb
+            _ = para.alignment
+
+    @patch("renderers.pptx_renderer.PptxPresentation")
+    def test_render_single_line_text_unchanged_behavior(self, mock_prs_class):
+        """测试单行文本行为不变（回归测试）"""
+        # 单行文本只需要 1 个段落
+        mock_slide = self._setup_mock_slide(num_paragraphs=1)
+        mock_prs_class.return_value = Mock()
+        mock_prs_class.return_value.slide_layouts = [None] * 7 + [Mock()]
+        mock_prs_class.return_value.slides.add_slide.return_value = mock_slide
+
+        gen = PptxGenerator()
+        # 单行文本
+        elem = TextElement(
+            content="单行文本",
+            box=[1, 2, 3, 1],
+            font={"size": 18},
+        )
+
+        gen._render_text(mock_slide, elem)
+
+        # 验证文本框被创建
+        mock_slide.shapes.add_textbox.assert_called_once()
+        # 验证字体大小被设置
+        mock_tf = mock_slide.shapes.add_textbox.return_value.text_frame
+        assert len(mock_tf.paragraphs) == 1
+        _ = mock_tf.paragraphs[0].font.size
+
+    def _setup_mock_slide(self, num_paragraphs=1):
+        """辅助函数：创建 mock slide
+
+        Args:
+            num_paragraphs: 要创建的段落数量，默认为 1
+        """
         mock_slide = Mock()
         mock_text_frame = Mock()
         mock_text_frame.word_wrap = True
-        mock_paragraph = Mock()
-        mock_paragraph.font = Mock()
-        mock_text_frame.paragraphs = [mock_paragraph]
+
+        # 创建指定数量的 mock 段落
+        mock_paragraphs = []
+        for _ in range(num_paragraphs):
+            mock_paragraph = Mock()
+            mock_paragraph.font = Mock()
+            mock_paragraphs.append(mock_paragraph)
+
+        mock_text_frame.paragraphs = mock_paragraphs
 
         mock_textbox = Mock()
         mock_textbox.text_frame = mock_text_frame