feat: 插件化 API 架构 — C API + 模块独立 + 外部集成示例

核心设计：
- include/vde/vde.h: 统一 C API（50+ 函数，6 种不透明句柄）
  - 上下文管理 / I/O / Mesh / 曲线 / 碰撞 / 约束求解 / B-Rep / 序列化
- src/capi/vde_capi.cpp: 完整 C API 实现 (240行)
- examples/07_capi_integration: C 语言外部调用示例
- CMake: 添加 vde_capi 库 + install 导出支持

模块架构（每个独立 .a）:
  vde_foundation → vde_core → vde_curves → vde_mesh
  → vde_spatial → vde_boolean / vde_collision
  → vde_brep / vde_sketch → vde_capi

examples/07_capi_integration/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_executable(capi_demo main.c)
+target_link_libraries(capi_demo PRIVATE vde_capi)

examples/07_capi_integration/main.c

@@ -0,0 +1,33 @@
+#include "vde/vde.h"
+#include <stdio.h>
+
+int main(void) {
+    VdeHandle ctx = vde_create();
+    printf("ViewDesignEngine %s\n", vde_version());
+
+    /* GJK spheres */
+    int hit = vde_collision_gjk_spheres(ctx, 0,0,0, 1.0, 1.5,0,0, 1.0);
+    printf("Spheres overlap: %s\n", hit ? "yes" : "no");
+
+    /* Constraint solver */
+    VdeSolverHandle s = vde_solver_create();
+    int p0 = vde_solver_add_point(s, 0,0,1);
+    int p1 = vde_solver_add_point(s, 3,0.5,0);
+    int l0 = vde_solver_add_line(s, p0, p1);
+    vde_solver_add_horizontal(s, l0);
+    vde_solver_add_distance_constraint(s, p0, p1, 5.0);
+    printf("Solver: %s\n", vde_solver_solve(s,100,1e-6) ? "ok" : "fail");
+    double x,y; vde_solver_get_point(s,p1,&x,&y);
+    printf(" P1=(%.4f,%.4f)\n", x, y);
+    vde_solver_free(s);
+
+    /* B-Rep box */
+    VdeBodyHandle box = vde_body_create_box(ctx, 2,2,2);
+    VdeMeshHandle m = vde_body_to_mesh(ctx, box, 0.01);
+    printf("Box: %zu faces\n", vde_mesh_num_faces(m));
+    vde_mesh_free(m);
+    vde_body_free(box);
+
+    vde_destroy(ctx);
+    return 0;
+}

examples/07_capi_integration/main.cpp

@@ -0,0 +1,41 @@
+#include <stdio.h>
+#include "vde/vde.h"
+
+int main(void) {
+    VdeHandle ctx = vde_create();
+    printf("ViewDesignEngine %s\n", vde_version());
+
+    /* ── Mesh I/O ── */
+    // VdeMeshHandle mesh = vde_load_mesh(ctx, "bunny.obj");
+    // printf("Loaded mesh: %zu vertices, %zu faces\n",
+    //        vde_mesh_num_vertices(mesh), vde_mesh_num_faces(mesh));
+
+    /* ── Collision: GJK spheres ── */
+    int hit = vde_collision_gjk_spheres(ctx, 0,0,0, 1.0, 1.5,0,0, 1.0);
+    printf("Spheres overlap: %s\n", hit ? "yes" : "no");
+
+    /* ── Sketch constraint solver ── */
+    VdeSolverHandle solver = vde_solver_create();
+    int p0 = vde_solver_add_point(solver, 0.0, 0.0, 1); /* fixed */
+    int p1 = vde_solver_add_point(solver, 3.0, 0.5, 0);
+    int l0 = vde_solver_add_line(solver, p0, p1);
+    vde_solver_add_horizontal(solver, l0);
+    vde_solver_add_distance_constraint(solver, p0, p1, 5.0);
+    int ok = vde_solver_solve(solver, 100, 1e-6);
+    printf("Solver converged: %s\n", ok ? "yes" : "no");
+    double x, y;
+    vde_solver_get_point(solver, p1, &x, &y);
+    printf("Point 1 after solve: (%.4f, %.4f)\n", x, y);
+    vde_solver_free(solver);
+
+    /* ── B-Rep: create box ── */
+    VdeBodyHandle box = vde_body_create_box(ctx, 2.0, 2.0, 2.0);
+    VdeMeshHandle box_mesh = vde_body_to_mesh(ctx, box, 0.01);
+    printf("Box mesh: %zu faces\n", vde_mesh_num_faces(box_mesh));
+    vde_mesh_free(box_mesh);
+    vde_body_free(box);
+
+    vde_destroy(ctx);
+    printf("Done.\n");
+    return 0;
+}

examples/CMakeLists.txt

@@ -4,3 +4,4 @@ add_subdirectory(03_mesh)
 add_subdirectory(04_delaunay)
 add_subdirectory(05_boolean)
 add_subdirectory(06_collision)
+add_subdirectory(07_capi_integration)