feat(runtime): 启动时扫描并接管已有窗口

从 handle_map_request 中提取 populate_window_event 供复用，
新增 backend_scan_windows 扫描 X11 根窗口子树，
runtime_scan 将扫描到的窗口走完整 policy 流程纳入管理。
route_map_request 增加 override_redirect 过滤。

src/core/backend.h

@@ -37,3 +37,17 @@ bool backend_apply_effect(
   const effect_t *effects,
   size_t effect_count
 );
+
+typedef struct backend_scan_result_t {
+  window_map_request_event_t *windows;
+  size_t count;
+  size_t capacity;
+} backend_scan_result_t;
+
+/**
+ * @brief 扫描已有窗口
+ *
+ * @details 仅扫描窗口并获取其基础信息
+ */
+backend_scan_result_t *backend_scan_windows(backend_t *backend);
+void backend_scan_result_destroy(backend_scan_result_t *result);