feat: scene graph - node tree + transform inheritance

- SceneNode: position/rotation(scale) + parent/child hierarchy
- World matrix: local = translate * rotate * scale, world = parent.world * local
- Euler/Quaternion rotation support
- Dirty flag propagation for efficient updates
- Scene init/find/create/destroy operations

Engine/Scene/scene_graph.h

@@ -0,0 +1,77 @@
+#pragma once
+
+// ============================================================================
+// scene_graph.h - 场景图: 节点树 + 变换继承
+// ============================================================================
+
+#include "../ASM/math_c_fallback.h"
+#include <stdint.h>
+
+#define MAX_CHILDREN 64
+#define MAX_NODES 1024
+
+// 场景节点
+typedef struct SceneNode {
+    int id;
+    char name[64];
+
+    // 本地变换
+    Vec3 position;
+    Quat rotation;
+    Vec3 scale;
+
+    // 世界矩阵 (每帧由 scene_update 计算)
+    Mat4 world_matrix;
+    Mat4 local_matrix;
+
+    // 层级
+    struct SceneNode* parent;
+    struct SceneNode* children[MAX_CHILDREN];
+    int child_count;
+
+    // 标志
+    int visible;
+    int dirty;  // 需要重新计算矩阵
+
+    // 用户数据
+    void* user_data;
+    int user_type;
+} SceneNode;
+
+// 场景
+typedef struct Scene {
+    SceneNode nodes[MAX_NODES];
+    int node_count;
+    SceneNode* root;
+    int next_id;
+} Scene;
+
+// 初始化/销毁
+void scene_init(Scene* scene);
+void scene_destroy(Scene* scene);
+
+// 节点管理
+SceneNode* scene_create_node(Scene* scene, const char* name);
+void scene_destroy_node(Scene* scene, SceneNode* node);
+SceneNode* scene_find_node(Scene* scene, int id);
+SceneNode* scene_find_by_name(Scene* scene, const char* name);
+
+// 层级
+void scene_set_parent(SceneNode* child, SceneNode* parent);
+void scene_add_child(SceneNode* parent, SceneNode* child);
+void scene_remove_child(SceneNode* parent, SceneNode* child);
+
+// 变换
+void scene_set_position(SceneNode* node, float x, float y, float z);
+void scene_set_rotation_euler(SceneNode* node, float rx, float ry, float rz);
+void scene_set_scale(SceneNode* node, float sx, float sy, float sz);
+void scene_set_rotation_quat(SceneNode* node, const Quat* q);
+void scene_mark_dirty(SceneNode* node);
+
+// 每帧更新
+void scene_update(Scene* scene);
+
+// 获取世界变换
+void scene_get_world_position(SceneNode* node, Vec3* out);
+void scene_get_world_rotation(SceneNode* node, Quat* out);
+void scene_get_world_scale(SceneNode* node, Vec3* out);