feat: Bezier 解析导数 + 测试补充 + 区间算术验证

- curves/bezier: 重写导数（前向差分 + de Casteljau）
- curves/bezier_surface: de Casteljau 解析导数 du/dv
- tests/bezier: 新增导数/分割测试 (5 cases)
- foundation/interval: 区间算术 + orient2d 验证函数

src/curves/bezier_curve.cpp

@@ -1,8 +1,12 @@
 #include "vde/curves/bezier_curve.h"
 #include <cmath>
+#include <algorithm>
 
 namespace vde::curves {
 
+using core::Point3D;
+using core::Vector3D;
+
 BezierCurve::BezierCurve(std::vector<Point3D> control_points)
     : cp_(std::move(control_points)) {}
 
@@ -20,32 +24,32 @@ Point3D BezierCurve::evaluate(double t) const {
 }
 
 Vector3D BezierCurve::derivative(double t, int order) const {
+    t = std::clamp(t, 0.0, 1.0);
     if (order < 1) return evaluate(t) - Point3D::Zero();
     int n = degree();
     if (order > n) return Vector3D::Zero();
-    std::vector<Point3D> dpts;
-    for (int i = 0; i <= n - order; ++i) {
-        Point3D sum = Point3D::Zero();
-        // Elevate via finite differences (simplified)
-        for (int j = 0; j <= order; ++j) {
-            double coef = (j % 2 == 0 ? 1.0 : -1.0);
-            // Binomial coefficient placeholder
-            int a = 1;
-            for (int k = 1; k <= order; ++k) a = a * (order - k + 1) / k;
-            if (j == 0 || j == order) coef *= 1;
-            else coef *= a;
-            sum += coef * cp_[i + order - j];
-        }
-        dpts.push_back(sum);
+
+    // Compute derivative via forward differences on control points
+    // P^(r)(t) = n*(n-1)*...*(n-r+1) * deCasteljau(Δ^r cp, t)
+    auto dcp = cp_;
+    for (int r = 0; r < order; ++r) {
+        std::vector<Point3D> diff;
+        int m = static_cast<int>(dcp.size()) - 1;
+        for (int i = 0; i < m; ++i)
+            diff.push_back(dcp[i+1] - dcp[i]);
+        dcp = std::move(diff);
     }
-    double fact = 1.0;
-    for (int i = 2; i <= order; ++i) fact *= (n - i + 1);
-    return fact * (de_casteljau(t, dpts) - Point3D::Zero());
+
+    double factor = 1.0;
+    for (int i = 0; i < order; ++i)
+        factor *= (n - i);
+
+    return factor * (de_casteljau(t, dcp) - Point3D::Zero());
 }
 
 std::pair<BezierCurve, BezierCurve> BezierCurve::split(double t) const {
     int n = degree();
-    std::vector<Point3D> left(n + 1), right(n + 1);
+    std::vector<Point3D> left(n+1), right(n+1);
     auto temp = cp_;
     left[0] = temp[0];
     right[n] = temp[n];
@@ -61,7 +65,7 @@ std::pair<BezierCurve, BezierCurve> BezierCurve::split(double t) const {
 BezierCurve BezierCurve::degree_elevate(int times) const {
     if (times <= 0) return *this;
     BezierCurve result = *this;
-    for (int t = 0; t < times; ++t) {
+    for (int t_i = 0; t_i < times; ++t_i) {
         int n = result.degree();
         std::vector<Point3D> new_cp(n + 2);
         new_cp[0] = result.cp_[0];