feat: GMP 精确算术后端 + 可插拔谓词接口

新增:
- include/vde/foundation/exact_predicates_gmp.h: GMP 精确谓词
  - orient_2d/3d, in_circle 完全精确（mpq_class 有理数）
  - 零舍入误差，任意精度
- include/vde/foundation/predicates.h: 统一谓词接口
  - Predicates<T> 编译期选择后端
  - VDE_USE_GMP=ON → GMP, OFF → 自适应 double
  - 便捷函数: orient(), in_circle()
- cmake/FindGMP.cmake: GMP 查找模块
- CMake: -DVDE_USE_GMP=ON 启用 GMP 精确模式
- AdaptiveDouble 包装器: 统一后端接口

使用:
  cmake -DVDE_USE_GMP=ON ..  # 完全精确模式

include/vde/foundation/exact_predicates.h

@@ -18,3 +18,20 @@ CircleTest in_circle(const Point2D& a, const Point2D& b,
                      const Point2D& c, const Point2D& d);
 
 }  // namespace vde::foundation::exact
+
+/// Adaptive double-precision predicates (default backend)
+namespace vde::foundation {
+struct AdaptiveDouble {
+    static exact::Orientation orient_2d(const Point2D& a, const Point2D& b, const Point2D& c) {
+        return exact::orient_2d(a, b, c);
+    }
+    static exact::Orientation orient_3d(const Point3D& a, const Point3D& b,
+                                  const Point3D& c, const Point3D& d) {
+        return exact::orient_3d(a, b, c, d);
+    }
+    static exact::CircleTest in_circle(const Point2D& a, const Point2D& b,
+                                const Point2D& c, const Point2D& d) {
+        return exact::in_circle(a, b, c, d);
+    }
+};
+}  // namespace vde::foundation