feat: 3D Delaunay + 多边形偏移 + 序列化

- mesh: Bowyer-Watson 3D Delaunay 四面体剖分
- boolean: 2D 多边形偏移（膨胀/腐蚀，角平分线法）
- foundation: 二进制序列化（版本化 Little-Endian 格式）
- 更新 CMake 构建

src/boolean/polygon_offset.cpp

@@ -0,0 +1,60 @@
+#include "vde/boolean/polygon_offset.h"
+#include <algorithm>
+#include <cmath>
+
+namespace vde::boolean {
+
+std::vector<Polygon2D> polygon_offset(const Polygon2D& poly, double distance) {
+    const auto& verts = poly.vertices();
+    if (verts.size() < 3 || std::abs(distance) < 1e-12) return {poly};
+
+    int n = static_cast<int>(verts.size());
+    std::vector<Point2D> result_verts;
+
+    // Offset each edge inward/outward by moving vertices along angle bisectors
+    for (int i = 0; i < n; ++i) {
+        const Point2D& prev = verts[(i - 1 + n) % n];
+        const Point2D& curr = verts[i];
+        const Point2D& next = verts[(i + 1) % n];
+
+        // Edge directions
+        Vector2D e1 = (curr - prev);
+        Vector2D e2 = (next - curr);
+        double len1 = e1.norm(), len2 = e2.norm();
+        if (len1 < 1e-12 || len2 < 1e-12) {
+            result_verts.push_back(curr);
+            continue;
+        }
+        e1 /= len1; e2 /= len2;
+
+        // Inward normals (rotate CW by 90°)
+        Vector2D n1(-e1.y(), e1.x());
+        Vector2D n2(-e2.y(), e2.x());
+
+        // Bisector direction
+        Vector2D bisector = n1 + n2;
+        double bisector_len = bisector.norm();
+
+        if (bisector_len < 1e-12) {
+            // Colinear edges — just offset along normal
+            result_verts.push_back(Point2D(curr.x() + distance * n1.x(),
+                                           curr.y() + distance * n1.y()));
+        } else {
+            bisector /= bisector_len;
+            // Scale: offset = distance / sin(angle/2)
+            double cos_angle = e1.dot(e2);
+            double sin_half_angle = std::sqrt(std::max(0.0, (1.0 - cos_angle) * 0.5));
+            if (sin_half_angle > 1e-12) {
+                double scale = distance / sin_half_angle;
+                result_verts.push_back(Point2D(curr.x() + scale * bisector.x(),
+                                               curr.y() + scale * bisector.y()));
+            } else {
+                result_verts.push_back(curr);
+            }
+        }
+    }
+
+    return {Polygon2D(result_verts)};
+}
+
+}  // namespace vde::boolean