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)

utils.py

@@ -0,0 +1,74 @@
+"""
+工具函数模块
+
+提供日志输出和颜色转换等通用功能。
+"""
+
+import sys
+
+
+# ============= 日志输出函数 =============
+
+def log_info(message):
+    """输出信息日志"""
+    print(f"[INFO] {message}")
+
+
+def log_success(message):
+    """输出成功日志"""
+    print(f"[SUCCESS] ✓ {message}")
+
+
+def log_error(message):
+    """输出错误日志"""
+    print(f"[ERROR] ✗ {message}", file=sys.stderr)
+
+
+def log_progress(current, total, message=""):
+    """输出进度日志"""
+    print(f"[PROGRESS] {current}/{total} {message}")
+
+
+# ============= 颜色转换函数 =============
+
+def hex_to_rgb(hex_color):
+    """
+    将十六进制颜色转换为 RGB 元组
+
+    Args:
+        hex_color: 十六进制颜色字符串，如 "#4a90e2" 或 "#fff"
+
+    Returns:
+        tuple: (R, G, B) 元组
+    """
+    hex_color = hex_color.lstrip('#')
+
+    # 处理短格式 #RGB -> #RRGGBB
+    if len(hex_color) == 3:
+        hex_color = ''.join([c*2 for c in hex_color])
+
+    if len(hex_color) != 6:
+        raise ValueError(f"无效的颜色格式: #{hex_color}")
+
+    try:
+        return tuple(int(hex_color[i:i+2], 16) for i in (0, 2, 4))
+    except ValueError:
+        raise ValueError(f"无效的颜色格式: #{hex_color}")
+
+
+def validate_color(color_value):
+    """
+    验证颜色值格式（十六进制 #RRGGBB 或 #RGB）
+
+    Args:
+        color_value: 颜色字符串
+
+    Returns:
+        bool: 是否有效
+    """
+    import re
+    if not isinstance(color_value, str):
+        return False
+    # 匹配 #RRGGBB 或 #RGB 格式
+    pattern = r'^#([0-9a-fA-F]{6}|[0-9a-fA-F]{3})$'
+    return re.match(pattern, color_value) is not None