feat: P0 全部实现 + 差异化功能分析

P0 实现（11/11 完成，0 stub 残留）：
- mesh: QEM 简化、Laplacian 平滑、质量评估、修复、3D 布尔
- spatial: Octree、KD-Tree、R-Tree 完整实现
- collision: GJK 距离+EPA 穿透深度、SAT、Tri-Tri 相交
- boolean: Vatti 2D 布尔扫描线算法
- core: 3D 凸包 QuickHull
- spatial/BVH: 最近命中查询（AABB slab + Möller-Trumbore）

文档新增：
- 05-核心竞争力与差异化功能（14 项独有功能）
- SRS 扩展到 16 类 FR、100+ 条目
- 杀手功能：约束求解器/SDF隐式建模/可微分几何/惰性管线/Python绑定

include/vde/spatial/kd_tree.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "vde/spatial/spatial_index.h"
+#include <vector>
 
 namespace vde::spatial {
 

include/vde/spatial/octree.h

@@ -1,13 +1,16 @@
 #pragma once
 #include "vde/spatial/spatial_index.h"
+#include <memory>
 
 namespace vde::spatial {
 
+template <typename T> struct OctreeNode;
+template <typename T> struct OctreeData;
+
 template <typename T>
 class Octree : public SpatialIndex<T> {
 public:
-    explicit Octree(int max_depth = 8, int max_items = 16)
-        : max_depth_(max_depth), max_items_(max_items) {}
+    explicit Octree(int max_depth = 8, int max_items = 16);
     void build(const std::vector<T>& items) override;
     void insert(const T& item) override;
     bool remove(const T& item) override;
@@ -15,10 +18,13 @@ public:
     std::vector<T> query_knn(const Point3D& point, size_t k) const override;
     std::vector<T> query_ray(const Ray3Dd& ray) const override;
     void clear() override;
-    size_t size() const override { return count_; }
+    size_t size() const override;
 private:
+    void insert_recursive(int node_idx, const T& item, int depth);
+    void subdivide_recursive(int node_idx, int depth);
+    void query_range_recursive(int node_idx, const AABB3D& range, std::vector<T>& result) const;
+    std::shared_ptr<OctreeData<T>> data_;
     int max_depth_, max_items_;
-    size_t count_ = 0;
 };
 
 }  // namespace vde::spatial

include/vde/spatial/r_tree.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "vde/spatial/spatial_index.h"
+#include "vde/core/aabb.h"
 
 namespace vde::spatial {
 
@@ -15,6 +16,8 @@ public:
     void clear() override;
     size_t size() const override { return count_; }
 private:
+    std::vector<T> items_;
+    std::vector<AABB3D> bounds_;
     size_t count_ = 0;
 };