feat(sdf): S11 — SDF 隐式建模完整模块

S11-A: SDF 图元库
- 14 种基础形状（inline header）
- sphere/box/round_box/torus/capsule/cylinder/cone/plane/ellipsoid
- triangular_prism/hex_prism/link/wedge
- 2D 挤出（extrusion/extrusion_bounded/revolution）

S11-B: SDF 操作 + 域变形
- 锐利布尔: union/intersection/difference
- 平滑布尔: smooth_union/smooth_intersection/smooth_difference
- 修饰器: round/onion
- 域变形: repeat/mirror/rotate/translate/scale/twist/bend/elongate/displace/cheap_bend
- 24 项测试

S11-C: CSG 表达式树 + 网格转换 + Python 绑定
- SdfNode 树结构 — 工厂构造函数，递归求值
- sdf_to_mesh — 基于 marching_cubes 的 SDF→网格
- bind_sdf — pybind11 Python 绑定

文件: 14 文件，2,545 行（测试 1,296 行）

include/vde/sdf/sdf_to_mesh.h

@@ -0,0 +1,29 @@
+#pragma once
+#include "vde/sdf/sdf_tree.h"
+#include "vde/mesh/marching_cubes.h"
+#include <functional>
+
+namespace vde::sdf {
+
+/// Convert an SDF expression tree to a triangle mesh using marching cubes
+/// @param root       SDF expression tree root node
+/// @param resolution Grid resolution per axis (e.g. 64 → 64³ grid)
+/// @param iso_level  Isosurface level (0 = surface, default)
+/// @return Mesh with vertices and triangle indices
+[[nodiscard]] mesh::MCMesh sdf_to_mesh(const SdfNodePtr& root,
+                                        int resolution = 64,
+                                        double iso_level = 0.0);
+
+/// Convert a lambda SDF f(x,y,z) to a triangle mesh
+/// Convenience overload for direct function binding (useful from Python)
+/// @param f          SDF function f(x,y,z) → signed distance
+/// @param bmin       Lower corner of bounding box
+/// @param bmax       Upper corner of bounding box
+/// @param resolution Grid resolution per axis
+/// @param iso_level  Isosurface level
+[[nodiscard]] mesh::MCMesh sdf_to_mesh_lambda(
+    const std::function<double(double, double, double)>& f,
+    const Point3D& bmin, const Point3D& bmax,
+    int resolution = 64, double iso_level = 0.0);
+
+} // namespace vde::sdf