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/conftest.py

@@ -95,14 +95,38 @@ elements:
       align: center
 """
 
+TEMPLATE_LIBRARY_YAML = """templates:
+  title-slide:
+    vars:
+      - name: title
+        required: true
+      - name: subtitle
+        required: false
+        default: ""
+    elements:
+      - type: text
+        box: [1, 2, 8, 1]
+        content: "{title}"
+        font:
+          size: 44
+          bold: true
+          align: center
+      - type: text
+        box: [1, 3.5, 8, 0.5]
+        content: "{subtitle}"
+        visible: "{subtitle != ''}"
+        font:
+          size: 24
+          align: center
+"""
+
 
 @pytest.fixture
 def sample_template(temp_dir):
-    """创建测试模板目录和文件"""
-    template_dir = temp_dir / "templates"
-    template_dir.mkdir()
-    (template_dir / "title-slide.yaml").write_text(TEMPLATE_YAML, encoding="utf-8")
-    return template_dir
+    """创建测试模板库文件"""
+    template_file = temp_dir / "templates.yaml"
+    template_file.write_text(TEMPLATE_LIBRARY_YAML, encoding="utf-8")
+    return template_file
 
 
 @pytest.fixture
@@ -156,65 +180,61 @@ def slide_size(request):
 def complex_template(temp_dir):
     """创建复杂模板（包含多个变量和条件）"""
     template_content = """
-vars:
-  - name: title
-    required: true
-  - name: subtitle
-    required: false
-    default: ""
-  - name: author
-    required: false
-    default: ""
-  - name: date
-    required: false
-    default: ""
-
-elements:
-  - type: shape
-    box: [0, 0, 10, 5.625]
-    shape: rectangle
-    fill: "#2c3e50"
-
-  - type: text
-    box: [1, 1.5, 8, 1]
-    content: "{title}"
-    font:
-      size: 44
-      bold: true
-      color: "#ffffff"
-      align: center
-
-  - type: text
-    box: [1, 2.8, 8, 0.6]
-    content: "{subtitle}"
-    visible: "{subtitle != ''}"
-    font:
-      size: 24
-      color: "#ecf0f1"
-      align: center
-
-  - type: text
-    box: [1, 4, 8, 0.5]
-    content: "{author}"
-    visible: "{author != ''}"
-    font:
-      size: 18
-      color: "#bdc3c7"
-      align: center
-
-  - type: text
-    box: [1, 4.8, 8, 0.4]
-    content: "{date}"
-    visible: "{date != ''}"
-    font:
-      size: 14
-      color: "#95a5a6"
-      align: center
+templates:
+  complex-slide:
+    vars:
+      - name: title
+        required: true
+      - name: subtitle
+        required: false
+        default: ""
+      - name: author
+        required: false
+        default: ""
+      - name: date
+        required: false
+        default: ""
+    elements:
+      - type: shape
+        box: [0, 0, 10, 5.625]
+        shape: rectangle
+        fill: "#2c3e50"
+      - type: text
+        box: [1, 1.5, 8, 1]
+        content: "{title}"
+        font:
+          size: 44
+          bold: true
+          color: "#ffffff"
+          align: center
+      - type: text
+        box: [1, 2.8, 8, 0.6]
+        content: "{subtitle}"
+        visible: "{subtitle != ''}"
+        font:
+          size: 24
+          color: "#ecf0f1"
+          align: center
+      - type: text
+        box: [1, 4, 8, 0.5]
+        content: "{author}"
+        visible: "{author != ''}"
+        font:
+          size: 18
+          color: "#bdc3c7"
+          align: center
+      - type: text
+        box: [1, 4.8, 8, 0.4]
+        content: "{date}"
+        visible: "{date != ''}"
+        font:
+          size: 14
+          color: "#95a5a6"
+          align: center
 """
-    template_dir = temp_dir / "templates"
-    template_dir.mkdir()
-    (template_dir / "complex-slide.yaml").write_text(template_content)
-    return template_dir
+    template_file = temp_dir / "complex_templates.yaml"
+    template_file.write_text(template_content)
+    return template_file
 
 
 @pytest.fixture