feat: v0.6.0 精度提升 — Shewchuk Level C + 分层容差 + 业内方案调研

精确谓词: Shewchuk Level B → Level C (expansion arithmetic)
  - Dekker split + Two-Product + Two-Sum + Expansion 4-component
  - orient_2d/3d 自适应精度，支持大坐标(>1e14)
  - in_circle 扩展精度

容差系统: 单级 → 分层
  - Tolerance::tighten/relax 继承
  - Tolerance::min 合并取更严格
  - ToleranceScope 作用域临时容差
  - 预设容差级别: Modeling/Fitting/Intersection/Snapping

文档: 新增 06-精度提升方案调研 (252行)
  - 6种主流精度方案对比
  - Parasolid/ACIS/OCCT/CGAL 商业内核解密
  - VDE v0.6-v2.0 精度提升路线图

include/vde/foundation/tolerance.h

@@ -2,9 +2,11 @@
 #include <Eigen/Core>
 #include <cmath>
 #include <limits>
+#include <vector>
 
 namespace vde::foundation {
 
+/// 分层容差：支持全局 + 局部覆盖
 class Tolerance {
 public:
     Tolerance(double absolute = 1e-6, double relative = 1e-8,
@@ -12,6 +14,7 @@ public:
         : absolute_(absolute), relative_(relative),
           angular_(angular), snapping_(snapping) {}
 
+    // ── 判定 ──
     bool points_equal(const Eigen::MatrixXd& a, const Eigen::MatrixXd& b) const {
         return (a - b).norm() < absolute_;
     }
@@ -31,12 +34,33 @@ public:
     double angular()  const { return angular_; }
     double snapping() const { return snapping_; }
 
-    void set_absolute(double v) { absolute_ = v; }
-    void set_relative(double v) { relative_ = v; }
+    // ── 分层容差：继承并收紧 ──
+    Tolerance tighten(double factor = 0.1) const {
+        return Tolerance(absolute_ * factor, relative_ * factor, angular_ * factor, snapping_ * factor);
+    }
 
+    Tolerance relax(double factor = 10.0) const {
+        return Tolerance(absolute_ * factor, relative_ * factor, angular_ * factor, snapping_ * factor);
+    }
+
+    // ── 合并：取更严格的那个 ──
+    static Tolerance min(const Tolerance& a, const Tolerance& b) {
+        return Tolerance(std::min(a.absolute_, b.absolute_),
+                        std::min(a.relative_, b.relative_),
+                        std::min(a.angular_, b.angular_),
+                        std::min(a.snapping_, b.snapping_));
+    }
+
+    // ── 全局实例 ──
     static Tolerance& global() { return global_; }
     static void set_global(const Tolerance& tol) { global_ = tol; }
 
+    // ── 默认建模容差等级 ──
+    static constexpr double Modeling()  { return 1e-6; }  // 建模级
+    static constexpr double Fitting()   { return 1e-3; }  // 拟合级
+    static constexpr double Intersection() { return 1e-8; } // 求交级
+    static constexpr double Snapping()  { return 1e-4; }  // 吸附级
+
 private:
     double absolute_;
     double relative_;
@@ -45,4 +69,15 @@ private:
     static Tolerance global_;
 };
 
+/// 局部容差堆栈：支持作用域内的临时容差
+class ToleranceScope {
+public:
+    explicit ToleranceScope(const Tolerance& tol) : previous_(Tolerance::global()) {
+        Tolerance::set_global(tol);
+    }
+    ~ToleranceScope() { Tolerance::set_global(previous_); }
+private:
+    Tolerance previous_;
+};
+
 }  // namespace vde::foundation