feat: OBJ model loader + fix mesh.h duplicates

- mesh_load_obj: parse vertices, UVs, faces from .obj files
- Auto-generate face normals
- Per-face vertex expansion (simple but correct)

Engine/Rendering/mesh.c

@@ -1,5 +1,7 @@
 #include "mesh.h"
 #include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
 #include <math.h>
 #ifndef MATH_ASM_AVAILABLE
 #include "../ASM/math_c_fallback.h"
@@ -108,3 +110,99 @@ Mesh* mesh_create_sphere(ID3D11Device* dev, float r, int seg) {
     free(v); free(idx);
     return m;
 }
+
+// OBJ 加载器
+Mesh* mesh_load_obj(ID3D11Device* dev, const char* path) {
+    FILE* f = fopen(path, "r");
+    if (!f) return NULL;
+
+    // 先统计顶点和面数
+    int max_v = 4096, max_f = 8192;
+    float* verts = (float*)malloc(max_v * 3 * sizeof(float));
+    float* uvs = (float*)malloc(max_v * 2 * sizeof(float));
+    int vert_count = 0, uv_count = 0;
+    int* face_idx = (int*)malloc(max_f * 3 * sizeof(int));
+    int face_count = 0;
+
+    char line[256];
+    while (fgets(line, sizeof(line), f)) {
+        if (line[0] == 'v' && line[1] == ' ') {
+            float x, y, z;
+            if (sscanf(line + 2, "%f %f %f", &x, &y, &z) == 3 && vert_count < max_v) {
+                verts[vert_count * 3] = x;
+                verts[vert_count * 3 + 1] = y;
+                verts[vert_count * 3 + 2] = z;
+                vert_count++;
+            }
+        } else if (line[0] == 'v' && line[1] == 't') {
+            float u, v;
+            if (sscanf(line + 3, "%f %f", &u, &v) == 2 && uv_count < max_v) {
+                uvs[uv_count * 2] = u;
+                uvs[uv_count * 2 + 1] = v;
+                uv_count++;
+            }
+        } else if (line[0] == 'f' && line[1] == ' ') {
+            // 解析面: f v/vt v/vt v/vt 或 f v v v
+            int v0, v1, v2;
+            char* p = line + 2;
+            v0 = atoi(p) - 1; while (*p && *p != ' ') p++; p++;
+            v1 = atoi(p) - 1; while (*p && *p != ' ') p++; p++;
+            v2 = atoi(p) - 1;
+            if (v0 >= 0 && v1 >= 0 && v2 >= 0 && face_count < max_f) {
+                face_idx[face_count * 3] = v0;
+                face_idx[face_count * 3 + 1] = v1;
+                face_idx[face_count * 3 + 2] = v2;
+                face_count++;
+            }
+        }
+    }
+    fclose(f);
+
+    if (vert_count == 0 || face_count == 0) {
+        free(verts); free(uvs); free(face_idx);
+        return NULL;
+    }
+
+    // 构建去重顶点 (简单方案: 每个面一个顶点)
+    int out_vcount = face_count * 3;
+    Vertex* out_verts = (Vertex*)calloc(out_vcount, sizeof(Vertex));
+    uint32_t* out_idx = (uint32_t*)malloc(face_count * 3 * sizeof(uint32_t));
+
+    for (int i = 0; i < face_count; i++) {
+        for (int j = 0; j < 3; j++) {
+            int vi = face_idx[i * 3 + j];
+            int oi = i * 3 + j;
+            if (vi < vert_count) {
+                out_verts[oi].pos[0] = verts[vi * 3];
+                out_verts[oi].pos[1] = verts[vi * 3 + 1];
+                out_verts[oi].pos[2] = verts[vi * 3 + 2];
+            }
+            out_verts[oi].color[0] = out_verts[oi].color[1] = out_verts[oi].color[2] = 1.0f;
+            out_verts[oi].color[3] = 1.0f;
+            out_idx[oi] = oi;
+        }
+    }
+
+    // 简单法线计算 (面法线)
+    for (int i = 0; i < face_count; i++) {
+        float* p0 = out_verts[i * 3].pos;
+        float* p1 = out_verts[i * 3 + 1].pos;
+        float* p2 = out_verts[i * 3 + 2].pos;
+        float e1[3] = {p1[0] - p0[0], p1[1] - p0[1], p1[2] - p0[2]};
+        float e2[3] = {p2[0] - p0[0], p2[1] - p0[1], p2[2] - p0[2]};
+        float nx = e1[1]*e2[2] - e1[2]*e2[1];
+        float ny = e1[2]*e2[0] - e1[0]*e2[2];
+        float nz = e1[0]*e2[1] - e1[1]*e2[0];
+        float nl = sqrtf(nx*nx + ny*ny + nz*nz);
+        if (nl > 0.0001f) { nx /= nl; ny /= nl; nz /= nl; }
+        for (int j = 0; j < 3; j++) {
+            out_verts[i * 3 + j].norm[0] = nx;
+            out_verts[i * 3 + j].norm[1] = ny;
+            out_verts[i * 3 + j].norm[2] = nz;
+        }
+    }
+
+    Mesh* m = create_mesh(dev, out_verts, out_vcount, out_idx, face_count * 3, sizeof(Vertex));
+    free(out_verts); free(out_idx); free(verts); free(uvs); free(face_idx);
+    return m;
+}

Engine/Rendering/mesh.h

@@ -20,5 +20,6 @@ typedef struct {
 Mesh* mesh_create_cube(ID3D11Device* dev);
 Mesh* mesh_create_plane(ID3D11Device* dev, float size);
 Mesh* mesh_create_sphere(ID3D11Device* dev, float r, int seg);
+Mesh* mesh_load_obj(ID3D11Device* dev, const char* path);
 void mesh_destroy(Mesh* m);
 void mesh_draw(Mesh* m, ID3D11DeviceContext* ctx);