feat: 为metadata、模板和幻灯片添加description字段支持

添加可选的description字段用于文档目的，不影响渲染输出。

主要更改：
- core/presentation.py: 添加metadata.description属性
- core/template.py: 添加template.description属性
- tests: 添加16个新测试用例验证description功能
- docs: 更新README.md和README_DEV.md文档
- specs: 新增page-description规范文件

tests/unit/test_template.py

@@ -8,6 +8,7 @@ import pytest
 from pathlib import Path
 from loaders.yaml_loader import YAMLError
 from core.template import Template
+from core.presentation import Presentation
 
 
 # ============= 模板初始化测试 =============
@@ -892,3 +893,165 @@ slides:
         slide_data = pres.data['slides'][0]
         rendered = pres.render_slide(slide_data)
         assert len(rendered['elements']) == 1
+
+
+# ============= Description 字段测试 =============
+
+
+class TestTemplateDescription:
+    """Template description 字段测试类"""
+
+    def test_template_with_description(self, temp_dir):
+        """测试模板包含 description 字段时正确加载"""
+        template_content = """
+description: "用于章节标题页的模板，包含主标题和副标题"
+vars:
+  - name: title
+    required: true
+  - name: subtitle
+    required: false
+    default: ""
+elements:
+  - type: text
+    content: "{title}"
+    box: [1, 1, 8, 1]
+    font:
+      size: 44
+      bold: true
+  - type: text
+    content: "{subtitle}"
+    box: [1, 2, 8, 1]
+    font:
+      size: 24
+"""
+        template_path = temp_dir / "test-template.yaml"
+        template_path.write_text(template_content)
+
+        template = Template("test-template", templates_dir=temp_dir)
+
+        assert template.description == "用于章节标题页的模板，包含主标题和副标题"
+
+    def test_template_without_description(self, sample_template):
+        """测试模板不包含 description 字段时正常工作"""
+        template = Template("title-slide", templates_dir=sample_template)
+
+        assert template.description is None
+
+    def test_template_description_empty_string(self, temp_dir):
+        """测试模板 description 为空字符串时正常工作"""
+        template_content = """
+description: ""
+vars:
+  - name: title
+elements:
+  - type: text
+    content: "{title}"
+    box: [1, 1, 8, 1]
+    font: {}
+"""
+        template_path = temp_dir / "test-template.yaml"
+        template_path.write_text(template_content)
+
+        template = Template("test-template", templates_dir=temp_dir)
+
+        assert template.description == ""
+
+    def test_template_description_chinese_characters(self, temp_dir):
+        """测试模板 description 包含中文字符时正确处理"""
+        template_content = """
+description: "这是中文模板描述，用于标题页面"
+vars:
+  - name: title
+elements:
+  - type: text
+    content: "{title}"
+    box: [1, 1, 8, 1]
+    font: {}
+"""
+        template_path = temp_dir / "test-template.yaml"
+        template_path.write_text(template_content)
+
+        template = Template("test-template", templates_dir=temp_dir)
+
+        assert "这是中文模板描述" in template.description
+        assert "标题页面" in template.description
+
+    def test_template_description_multiline(self, temp_dir):
+        """测试模板 description 支持多行文本"""
+        template_content = """
+description: |
+  这是一个多行描述。
+  第一行说明模板的用途。
+  第二行说明使用场景。
+vars:
+  - name: title
+elements:
+  - type: text
+    content: "{title}"
+    box: [1, 1, 8, 1]
+    font: {}
+"""
+        template_path = temp_dir / "test-template.yaml"
+        template_path.write_text(template_content)
+
+        template = Template("test-template", templates_dir=temp_dir)
+
+        # 多行文本应该被正确读取
+        assert "这是一个多行描述" in template.description
+        assert "第一行说明模板的用途" in template.description
+        assert "第二行说明使用场景" in template.description
+
+    def test_inline_template_with_description(self, temp_dir):
+        """测试内联模板包含 description 字段"""
+        yaml_content = """
+metadata:
+  size: "16:9"
+
+templates:
+  test-template:
+    description: "内联模板描述"
+    vars:
+      - name: title
+    elements:
+      - type: text
+        content: "{title}"
+        box: [1, 1, 8, 1]
+        font: {}
+
+slides:
+  - template: test-template
+    vars:
+      title: "Test"
+"""
+        yaml_path = temp_dir / "test.yaml"
+        yaml_path.write_text(yaml_content)
+
+        pres = Presentation(str(yaml_path))
+        template = pres.get_template("test-template")
+
+        assert template.description == "内联模板描述"
+
+    def test_template_description_does_not_affect_rendering(self, temp_dir):
+        """测试 description 不影响模板渲染"""
+        template_content = """
+description: "这段描述不应该影响渲染"
+vars:
+  - name: title
+elements:
+  - type: text
+    content: "{title}"
+    box: [1, 1, 8, 1]
+    font:
+      size: 44
+"""
+        template_path = temp_dir / "test-template.yaml"
+        template_path.write_text(template_content)
+
+        template = Template("test-template", templates_dir=temp_dir)
+
+        # 渲染应该正常工作，description 不影响结果
+        result = template.render({"title": "Test Title"})
+
+        assert len(result) == 1
+        assert result[0]["content"] == "Test Title"
+