refactor: modularize yaml2pptx into layered architecture

Refactor yaml2pptx.py from a 1,245-line monolithic script into a modular
architecture with clear separation of concerns. The entry point is now
127 lines, with business logic distributed across focused modules.

Architecture:
- core/: Domain models (elements, template, presentation)
- loaders/: YAML loading and validation
- renderers/: PPTX and HTML rendering
- preview/: Flask preview server
- utils.py: Shared utilities

Key improvements:
- Element abstraction layer using dataclass with validation
- Renderer logic built into generator classes
- Single-direction dependencies (no circular imports)
- Each module 150-300 lines for better readability
- Backward compatible CLI interface

Documentation:
- README.md: User-facing usage guide
- README_DEV.md: Developer documentation

OpenSpec:
- Archive refactor-yaml2pptx-modular change (63/70 tasks complete)
- Sync 5 delta specs to main specs (2 new + 3 updated)

loaders/yaml_loader.py

@@ -0,0 +1,113 @@
+"""
+YAML 加载和验证模块
+
+负责加载 YAML 文件并验证其结构。
+"""
+
+from pathlib import Path
+import yaml
+
+
+# ============= YAML 解析和验证 =============
+
+class YAMLError(Exception):
+    """YAML 相关错误"""
+    pass
+
+
+def load_yaml_file(file_path):
+    """
+    加载 YAML 文件（UTF-8 编码，错误处理）
+
+    Args:
+        file_path: 文件路径（字符串或 Path 对象）
+
+    Returns:
+        解析后的 Python 字典
+
+    Raises:
+        YAMLError: 文件不存在、权限不足、YAML 语法错误等
+    """
+    file_path = Path(file_path)
+
+    # 检查文件是否存在
+    if not file_path.exists():
+        raise YAMLError(f"文件不存在: {file_path}")
+
+    # 检查是否有读取权限
+    if not file_path.is_file():
+        raise YAMLError(f"不是有效的文件: {file_path}")
+
+    try:
+        with open(file_path, 'r', encoding='utf-8') as f:
+            data = yaml.safe_load(f)
+            return data
+    except PermissionError:
+        raise YAMLError(f"权限不足，无法读取文件: {file_path}")
+    except yaml.YAMLError as e:
+        # 提取行号信息
+        if hasattr(e, 'problem_mark'):
+            mark = e.problem_mark
+            raise YAMLError(
+                f"YAML 语法错误: {file_path}, 第 {mark.line + 1} 行: {e.problem}"
+            )
+        else:
+            raise YAMLError(f"YAML 解析错误: {file_path}: {str(e)}")
+    except Exception as e:
+        raise YAMLError(f"读取文件失败: {file_path}: {str(e)}")
+
+
+def validate_presentation_yaml(data, file_path=""):
+    """
+    验证演示文稿 YAML 结构（必需字段：slides）
+
+    Args:
+        data: 解析后的 YAML 数据
+        file_path: 文件路径（用于错误消息）
+
+    Raises:
+        YAMLError: 结构验证失败
+    """
+    if not isinstance(data, dict):
+        raise YAMLError(f"{file_path}: 演示文稿必须是一个字典对象")
+
+    # 验证 slides 字段
+    if 'slides' not in data:
+        raise YAMLError(f"{file_path}: 缺少必需字段 'slides'")
+
+    if not isinstance(data['slides'], list):
+        raise YAMLError(f"{file_path}: 'slides' 必须是一个列表")
+
+
+def validate_template_yaml(data, file_path=""):
+    """
+    验证模板 YAML 结构（vars, elements）
+
+    Args:
+        data: 解析后的 YAML 数据
+        file_path: 文件路径（用于错误消息）
+
+    Raises:
+        YAMLError: 结构验证失败
+    """
+    if not isinstance(data, dict):
+        raise YAMLError(f"{file_path}: 模板必须是一个字典对象")
+
+    # 验证 vars 字段
+    if 'vars' in data:
+        if not isinstance(data['vars'], list):
+            raise YAMLError(f"{file_path}: 'vars' 必须是一个列表")
+
+        # 验证每个变量定义
+        for i, var_def in enumerate(data['vars']):
+            if not isinstance(var_def, dict):
+                raise YAMLError(f"{file_path}: vars[{i}] 必须是一个字典对象")
+            if 'name' not in var_def:
+                raise YAMLError(f"{file_path}: vars[{i}] 缺少必需字段 'name'")
+
+    # 验证 elements 字段
+    if 'elements' not in data:
+        raise YAMLError(f"{file_path}: 缺少必需字段 'elements'")
+
+    if not isinstance(data['elements'], list):
+        raise YAMLError(f"{file_path}: 'elements' 必须是一个列表")