feat: P2-P4 全面补完 — B-Rep建模 + 数据交换 + 约束求解器 + ICP

P2 B-Rep建模:
- brep: fillet/chamfer/shell 完整实现
- brep: box/cylinder/sphere 实体工厂
- brep: extrude/revolve/sweep/loft 建模操作

P3 数据交换:
- foundation: PLY 读写（ASCII+二进制）
- foundation: glTF 2.0 导出（JSON+bin）

P4 高级算法/杀手功能:
- sketch: 2D 草图约束求解器（Newton-Raphson, 9种约束类型）
- core: ICP 点云配准（SVD最优变换+KD-Tree加速）

新增模块: vde_sketch

include/vde/sketch/constraint_solver.h

@@ -0,0 +1,48 @@
+#pragma once
+#include "vde/core/point.h"
+#include <vector>
+#include <string>
+
+namespace vde::sketch {
+
+struct SketchPoint { int id; Point2D pos; bool fixed = false; };
+struct SketchLine { int id, p0, p1; };
+struct SketchCircle { int id, center; double radius; };
+
+enum class ConstraintType {
+    Horizontal, Vertical, Parallel, Perpendicular, Coincident,
+    EqualLength, Distance, Radius, Tangent, Concentric, FixedPoint
+};
+
+struct Constraint {
+    ConstraintType type;
+    std::vector<int> elements;
+    double value = 0.0;
+};
+
+struct SolverResult {
+    bool converged = false; int iterations = 0; double residual = 0.0;
+    std::vector<Point2D> points; std::string message;
+};
+
+class ConstraintSolver {
+public:
+    int add_point(double x, double y, bool fixed = false);
+    int add_line(int p0, int p1);
+    int add_circle(int center, double radius);
+    void add_constraint(ConstraintType type, const std::vector<int>& elements, double val = 0.0);
+    void fix_point(int pid) { if(pid>=0&&pid<(int)points_.size()) points_[pid].fixed=true; }
+
+    [[nodiscard]] SolverResult solve(int max_iter = 100, double tol = 1e-8);
+    [[nodiscard]] int degrees_of_freedom() const;
+    [[nodiscard]] const std::vector<SketchPoint>& points() const { return points_; }
+
+private:
+    std::vector<SketchPoint> points_;
+    std::vector<SketchLine> lines_;
+    std::vector<SketchCircle> circles_;
+    std::vector<Constraint> constraints_;
+    double eval_constraint(const Constraint& c) const;
+};
+
+}  // namespace vde::sketch