feat: v0.5.0 P1 + 差异化功能 — Voronoi, Marching Cubes, 曲率, 序列化, Python绑定

新增功能：
- core: 2D Voronoi 图（Delaunay 对偶图）
- mesh: Marching Cubes（完整256项三角表 + SDF 辅助函数）
- mesh: 离散曲率（Gaussian/Mean, Cotan 公式）
- foundation: 二进制序列化（版本化格式，Little-Endian）
- python: pybind11 绑定（30+ API 暴露）
- marching_cubes.h: SDF 基本体（球/盒）+ 光滑布尔

include/vde/mesh/marching_cubes.h

@@ -0,0 +1,47 @@
+#pragma once
+#include "vde/core/point.h"
+#include <vector>
+#include <array>
+#include <functional>
+
+namespace vde::mesh {
+
+struct MCMesh {
+    std::vector<Point3D> vertices;
+    std::vector<std::array<int, 3>> triangles;
+};
+
+/// Marching Cubes for scalar field f(x,y,z)
+/// Extracts isosurface at value = iso_level within the bounding box
+MCMesh marching_cubes(const std::function<double(double,double,double)>& f,
+                      double iso_level,
+                      const Point3D& bmin, const Point3D& bmax,
+                      int resolution);
+
+/// SDF sphere
+inline double sdf_sphere(double x, double y, double z, double cx, double cy, double cz, double r) {
+    return std::sqrt((x-cx)*(x-cx) + (y-cy)*(y-cy) + (z-cz)*(z-cz)) - r;
+}
+
+/// SDF box
+inline double sdf_box(double x, double y, double z, double hx, double hy, double hz) {
+    double dx = std::abs(x) - hx, dy = std::abs(y) - hy, dz = std::abs(z) - hz;
+    return std::sqrt(std::max(dx,0.0)*std::max(dx,0.0)
+                   + std::max(dy,0.0)*std::max(dy,0.0)
+                   + std::max(dz,0.0)*std::max(dz,0.0))
+         + std::min(std::max({dx,dy,dz}), 0.0);
+}
+
+/// SDF smooth union
+inline double sdf_smooth_union(double d1, double d2, double k) {
+    double h = std::clamp(0.5 + 0.5*(d2-d1)/k, 0.0, 1.0);
+    return std::lerp(d2, d1, h) - k*h*(1.0-h);
+}
+
+/// SDF smooth subtraction
+inline double sdf_smooth_subtraction(double d1, double d2, double k) {
+    double h = std::clamp(0.5 - 0.5*(d2+d1)/k, 0.0, 1.0);
+    return std::lerp(d2, -d1, h) + k*h*(1.0-h);
+}
+
+}  // namespace vde::mesh