refactor: 重构外部模板系统，改为单文件模板库模式

主要变更：
- 将 templates_dir 参数改为 template_file，支持单个模板库 YAML 文件
- 添加模板库 YAML 验证功能
- 为模板添加 base_dir 支持，正确解析相对路径资源
- 内联模板与外部模板同名时改为警告（内联优先）
- 移除模板缓存机制，直接使用模板库字典
- 更新所有相关测试以适配新的模板加载方式

此重构简化了模板管理，使模板资源的路径解析更加清晰明确。

tests/unit/test_presentation.py

@@ -23,18 +23,18 @@ class TestPresentationInit:
         assert pres.pres_file == sample_yaml
 
     def test_init_with_templates_dir(self, sample_yaml, sample_template):
-        """测试带模板目录初始化"""
+        """测试带模板库文件初始化"""
         pres = Presentation(str(sample_yaml), str(sample_template))
 
-        assert pres.templates_dir == str(sample_template)
-        assert isinstance(pres.template_cache, dict)
+        assert pres.template_file == Path(sample_template)
+        assert pres.template_library is not None
 
     def test_init_without_templates_dir(self, sample_yaml):
-        """测试不带模板目录初始化"""
+        """测试不带模板库文件初始化"""
         pres = Presentation(str(sample_yaml))
 
-        assert pres.templates_dir is None
-        assert isinstance(pres.template_cache, dict)
+        assert pres.template_file is None
+        assert pres.template_library is None
 
     @patch("core.presentation.load_yaml_file")
     @patch("core.presentation.validate_presentation_yaml")
@@ -118,67 +118,6 @@ slides:
         assert pres.size == "16:9"
 
 
-class TestGetTemplate:
-    """get_template 方法测试类"""
-
-    @patch("core.presentation.Template")
-    def test_get_template_caches_template(self, mock_template_class, sample_template):
-        """测试模板被缓存"""
-        mock_template = Mock()
-        mock_template_class.return_value = mock_template
-
-        # 创建一个使用模板的 YAML
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = sample_template / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        # 第一次获取
-        template1 = pres.get_template("test_template")
-        # 第二次获取
-        template2 = pres.get_template("test_template")
-
-        # 应该是同一个实例
-        assert template1 is template2
-
-    @patch("core.presentation.Template")
-    def test_get_template_creates_new_template(
-        self, mock_template_class, sample_template
-    ):
-        """测试创建新模板"""
-        mock_template = Mock()
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = sample_template / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        # 获取模板
-        template = pres.get_template("new_template")
-
-        # 应该创建模板
-        mock_template_class.assert_called_once()
-        assert template == mock_template
-
-    def test_get_template_without_templates_dir(self, sample_yaml):
-        """测试无模板目录时获取模板"""
-        pres = Presentation(str(sample_yaml))
-
-        # 应该在调用 Template 时失败，而不是 get_template
-        with patch("core.presentation.Template") as mock_template_class:
-            mock_template_class.side_effect = YAMLError("No template dir")
-
-            with pytest.raises(YAMLError):
-                pres.get_template("test")
 
 
 class TestRenderSlide:
@@ -200,38 +139,6 @@ class TestRenderSlide:
         assert len(result["elements"]) == 1
         assert result["elements"][0].content == "Test"
 
-    @patch("core.presentation.Template")
-    def test_render_slide_with_template(
-        self, mock_template_class, temp_dir, sample_template
-    ):
-        """测试渲染使用模板的幻灯片"""
-        mock_template = Mock()
-        mock_template.render.return_value = [
-            {
-                "type": "text",
-                "content": "Template Title",
-                "box": [0, 0, 1, 1],
-                "font": {},
-            }
-        ]
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {"template": "title-slide", "vars": {"title": "My Title"}}
-
-        result = pres.render_slide(slide_data)
-
-        # 模板应该被渲染
-        mock_template.render.assert_called_once_with({"title": "My Title"})
-        assert "elements" in result
 
     def test_render_slide_with_background(self, sample_yaml):
         """测试渲染带背景的幻灯片"""
@@ -275,35 +182,6 @@ slides:
         mock_create_element.assert_called()
         assert result["elements"][0] == mock_elem
 
-    def test_render_slide_with_template_merges_background(
-        self, mock_template_class, temp_dir, sample_template
-    ):
-        """测试使用模板时合并背景"""
-        mock_template = Mock()
-        mock_template.render.return_value = [
-            {"type": "text", "content": "Title", "box": [0, 0, 1, 1], "font": {}}
-        ]
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {
-            "template": "test",
-            "vars": {},
-            "background": {"color": "#ff0000"},
-        }
-
-        result = pres.render_slide(slide_data)
-
-        # 背景应该被保留
-        assert result["background"] == {"color": "#ff0000"}
 
     def test_render_slide_empty_elements_list(self, sample_yaml):
         """测试空元素列表"""
@@ -341,79 +219,6 @@ slides:
 class TestRenderSlideHybridMode:
     """render_slide 混合模式测试类"""
 
-    @patch("core.presentation.Template")
-    def test_hybrid_mode_basic(self, mock_template_class, temp_dir, sample_template):
-        """测试混合模式基本功能：template + elements"""
-        mock_template = Mock()
-        mock_template.render.return_value = [
-            {"type": "text", "content": "From Template", "box": [0, 0, 1, 1], "font": {}}
-        ]
-        mock_template.resolve_element.side_effect = lambda elem, vars: elem
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {
-            "template": "test-template",
-            "vars": {"title": "Test"},
-            "elements": [
-                {"type": "text", "content": "Custom Element", "box": [2, 2, 1, 1], "font": {}}
-            ]
-        }
-
-        result = pres.render_slide(slide_data)
-
-        # 应该有 2 个元素：1 个来自模板，1 个自定义
-        assert len(result["elements"]) == 2
-        assert result["elements"][0].content == "From Template"
-        assert result["elements"][1].content == "Custom Element"
-
-    @patch("core.presentation.Template")
-    def test_hybrid_mode_variable_sharing(self, mock_template_class, temp_dir, sample_template):
-        """测试自定义元素访问模板变量"""
-        mock_template = Mock()
-        mock_template.render.return_value = []
-
-        # 模拟 resolve_element 解析变量
-        def resolve_element_mock(elem, vars):
-            resolved = elem.copy()
-            if "content" in resolved and "{" in resolved["content"]:
-                resolved["content"] = resolved["content"].replace("{theme_color}", vars.get("theme_color", ""))
-            if "fill" in resolved and "{" in resolved["fill"]:
-                resolved["fill"] = resolved["fill"].replace("{theme_color}", vars.get("theme_color", ""))
-            return resolved
-
-        mock_template.resolve_element.side_effect = resolve_element_mock
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {
-            "template": "test-template",
-            "vars": {"theme_color": "#3949ab"},
-            "elements": [
-                {"type": "shape", "fill": "{theme_color}", "box": [0, 0, 1, 1]}
-            ]
-        }
-
-        result = pres.render_slide(slide_data)
-
-        # 自定义元素应该使用模板变量
-        assert result["elements"][0].fill == "#3949ab"
 
     def test_hybrid_mode_empty_elements(self, temp_dir, sample_template):
         """测试空 elements 列表"""
@@ -529,84 +334,6 @@ templates:
         assert result["elements"][0].content == "Inline Template"
         assert result["elements"][1].content == "Custom"
 
-    @patch("core.presentation.Template")
-    def test_hybrid_mode_with_external_template(self, mock_template_class, temp_dir, sample_template):
-        """测试外部模板与自定义元素混合使用"""
-        mock_template = Mock()
-        mock_template.render.return_value = [
-            {"type": "text", "content": "External", "box": [0, 0, 1, 1], "font": {}}
-        ]
-        mock_template.resolve_element.side_effect = lambda elem, vars: elem
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {
-            "template": "external-template",
-            "vars": {},
-            "elements": [
-                {"type": "text", "content": "Custom", "box": [2, 2, 1, 1], "font": {}}
-            ]
-        }
-
-        result = pres.render_slide(slide_data)
-
-        # 应该有 2 个元素
-        assert len(result["elements"]) == 2
-        assert result["elements"][0].content == "External"
-        assert result["elements"][1].content == "Custom"
-
-    @patch("core.presentation.Template")
-    def test_hybrid_mode_element_order(self, mock_template_class, temp_dir, sample_template):
-        """测试元素顺序：模板元素在前，自定义元素在后"""
-        mock_template = Mock()
-        mock_template.render.return_value = [
-            {"type": "text", "content": "Template1", "box": [0, 0, 1, 1], "font": {}},
-            {"type": "text", "content": "Template2", "box": [1, 0, 1, 1], "font": {}}
-        ]
-        mock_template.resolve_element.side_effect = lambda elem, vars: elem
-        mock_template_class.return_value = mock_template
-
-        yaml_content = """
-slides:
-  - elements: []
-"""
-        yaml_path = temp_dir / "test.yaml"
-        yaml_path.write_text(yaml_content)
-
-        pres = Presentation(str(yaml_path), str(sample_template))
-
-        slide_data = {
-            "template": "test-template",
-            "vars": {},
-            "elements": [
-                {"type": "text", "content": "Custom1", "box": [2, 0, 1, 1], "font": {}},
-                {"type": "text", "content": "Custom2", "box": [3, 0, 1, 1], "font": {}}
-            ]
-        }
-
-        result = pres.render_slide(slide_data)
-
-        # 验证顺序：模板元素在前
-        assert len(result["elements"]) == 4
-        assert result["elements"][0].content == "Template1"
-        assert result["elements"][1].content == "Template2"
-        assert result["elements"][2].content == "Custom1"
-        assert result["elements"][3].content == "Custom2"
-
-
-# ============= Description 字段测试 =============
-
-
-class TestPresentationDescription:
-    """Presentation description 字段测试类"""
 
     def test_metadata_with_description(self, temp_dir):
         """测试 metadata 包含 description 字段时正确加载"""