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)

core/elements.py

@@ -0,0 +1,96 @@
+"""
+元素抽象层模块
+
+定义统一的元素数据类，支持元素验证和未来扩展。
+"""
+
+from dataclasses import dataclass, field
+from typing import Optional
+
+
+@dataclass
+class TextElement:
+    """文本元素"""
+    type: str = 'text'
+    content: str = ''
+    box: list = field(default_factory=lambda: [1, 1, 8, 1])
+    font: dict = field(default_factory=dict)
+
+    def __post_init__(self):
+        """创建时验证"""
+        if not isinstance(self.box, list) or len(self.box) != 4:
+            raise ValueError("box 必须是包含 4 个数字的列表")
+
+
+@dataclass
+class ImageElement:
+    """图片元素"""
+    type: str = 'image'
+    src: str = ''
+    box: list = field(default_factory=lambda: [1, 1, 4, 3])
+
+    def __post_init__(self):
+        """创建时验证"""
+        if not self.src:
+            raise ValueError("图片元素必须指定 src")
+        if not isinstance(self.box, list) or len(self.box) != 4:
+            raise ValueError("box 必须是包含 4 个数字的列表")
+
+
+@dataclass
+class ShapeElement:
+    """形状元素"""
+    type: str = 'shape'
+    shape: str = 'rectangle'
+    box: list = field(default_factory=lambda: [1, 1, 2, 1])
+    fill: Optional[str] = None
+    line: Optional[dict] = None
+
+    def __post_init__(self):
+        """创建时验证"""
+        if not isinstance(self.box, list) or len(self.box) != 4:
+            raise ValueError("box 必须是包含 4 个数字的列表")
+
+
+@dataclass
+class TableElement:
+    """表格元素"""
+    type: str = 'table'
+    data: list = field(default_factory=list)
+    position: list = field(default_factory=lambda: [1, 1])
+    col_widths: list = field(default_factory=list)
+    style: dict = field(default_factory=dict)
+
+    def __post_init__(self):
+        """创建时验证"""
+        if not self.data:
+            raise ValueError("表格数据不能为空")
+        if not isinstance(self.position, list) or len(self.position) != 2:
+            raise ValueError("position 必须是包含 2 个数字的列表")
+
+
+def create_element(elem_dict: dict):
+    """
+    元素工厂函数，从字典创建对应类型的元素对象
+
+    Args:
+        elem_dict: 元素字典，必须包含 type 字段
+
+    Returns:
+        对应类型的元素对象
+
+    Raises:
+        ValueError: 不支持的元素类型
+    """
+    elem_type = elem_dict.get('type')
+
+    if elem_type == 'text':
+        return TextElement(**elem_dict)
+    elif elem_type == 'image':
+        return ImageElement(**elem_dict)
+    elif elem_type == 'shape':
+        return ShapeElement(**elem_dict)
+    elif elem_type == 'table':
+        return TableElement(**elem_dict)
+    else:
+        raise ValueError(f"不支持的元素类型: {elem_type}")