feat: 3D Delaunay + 多边形偏移 + 序列化

- mesh: Bowyer-Watson 3D Delaunay 四面体剖分
- boolean: 2D 多边形偏移（膨胀/腐蚀，角平分线法）
- foundation: 二进制序列化（版本化 Little-Endian 格式）
- 更新 CMake 构建

include/vde/boolean/polygon_offset.h

@@ -0,0 +1,11 @@
+#pragma once
+#include "vde/core/polygon.h"
+#include <vector>
+
+namespace vde::boolean {
+
+/// Offset a polygon by distance (positive = inflate, negative = deflate)
+/// Uses straight skeleton approximation via edge normal displacement
+std::vector<Polygon2D> polygon_offset(const Polygon2D& poly, double distance);
+
+}  // namespace vde::boolean