feat(v3.3): NURBS surface-surface intersection

Implement industrial-grade NURBS surface-surface intersection algorithm:

Phase 1 — Recursive AABB subdivision with overlap culling
Phase 2 — Grid-based seed point discovery in leaf patches
Phase 3 — Newton refinement + tangent tracing along intersection curves
Phase 4 — Chord-length NURBS curve fitting through traced points

Also implement plane-surface intersection using contour tracing
on the parameter domain with Newton refinement.

New files:
- include/vde/curves/surface_intersection.h  (IntersectionCurve struct, APIs)
- src/curves/surface_intersection.cpp        (full algorithm implementation)
- tests/curves/test_surface_intersection.cpp (16 test cases)

Tests cover: plane-cylinder (horizontal/vertical/oblique cuts),
plane-planar, two intersecting planes, self-intersection,
non-intersecting surfaces, sphere-cylinder, fitted curve
verification, and robustness edge cases.

include/vde/curves/surface_intersection.h

@@ -0,0 +1,83 @@
+#pragma once
+#include "vde/curves/nurbs_surface.h"
+#include "vde/curves/nurbs_curve.h"
+#include "vde/core/aabb.h"
+#include <vector>
+#include <functional>
+#include <optional>
+#include <utility>
+
+namespace vde::curves {
+
+/**
+ * @brief 曲面-曲面求交结果
+ *
+ * 包含交线上离散采样点的 3D 坐标及其在两曲面上的参数坐标，
+ * 以及可选的拟合 NURBS 曲线。
+ *
+ * @ingroup curves
+ */
+struct IntersectionCurve {
+    /// 交线采样点（3D 空间坐标）
+    std::vector<core::Point3D> points;
+    /// 各采样点在曲面 A 上的参数坐标 (u1, v1)
+    std::vector<std::pair<double, double>> params_a;
+    /// 各采样点在曲面 B 上的参数坐标 (u2, v2)
+    std::vector<std::pair<double, double>> params_b;
+    /// 沿交线采样点拟合的 NURBS 曲线（可选）
+    std::optional<NurbsCurve> curve;
+};
+
+/**
+ * @brief 两 NURBS 曲面求交
+ *
+ * 算法：递归子分 AABB 剔除 → 叶节点种子搜索 → Newton 精化 + 交线追踪 →
+ *       弦长参数化 NURBS 拟合。
+ *
+ * @param surf_a 曲面 A
+ * @param surf_b 曲面 B
+ * @param max_depth 最大递归深度（默认 5）
+ * @param tolerance 3D 空间收敛容差（默认 1e-4）
+ * @return 交线列表（无交则空）
+ *
+ * @note 本实现为工业级近似算法，适用于一般 CAD 场景。
+ *       对奇异情况（切线接触、边界交线等）可能丢线。
+ * @code{.cpp}
+ * auto curves = intersect_surfaces(cyl, sphere, 6, 1e-5);
+ * for (auto& c : curves) {
+ *     for (auto& p : c.points) { /* 使用交线上的点 */ }
+ * }
+ * @endcode
+ * @ingroup curves
+ */
+[[nodiscard]] std::vector<IntersectionCurve> intersect_surfaces(
+    const NurbsSurface& surf_a,
+    const NurbsSurface& surf_b,
+    int max_depth = 5,
+    double tolerance = 1e-4);
+
+/**
+ * @brief 平面与 NURBS 曲面求交
+ *
+ * 使用等值线追踪法：在曲面参数域上采样 signed_distance，
+ * 追踪零等值线得到交线。
+ *
+ * @param surf NURBS 曲面
+ * @param point 截平面上一点
+ * @param normal 截平面法向（自动归一化）
+ * @param grid_res 参数域网格分辨率（默认 32）
+ * @return 交线列表（通常为一条）
+ *
+ * @code{.cpp}
+ * // 用 Z=0 平面截圆柱
+ * auto curves = intersect_plane_surface(cylinder, {0,0,0}, {0,0,1});
+ * @endcode
+ * @ingroup curves
+ */
+[[nodiscard]] std::vector<IntersectionCurve> intersect_plane_surface(
+    const NurbsSurface& surf,
+    const core::Point3D& point,
+    const core::Vector3D& normal,
+    int grid_res = 32);
+
+} // namespace vde::curves