图库: 修系统返回语义与打开册资源路径（M15）

- 返回手势：书库列表排除左缘 24px 系统返回带并清空残留拖拽态；library_go_up
  到 TF 根返回 0（app.c 退出 App），不再上溯系统目录；顶栏返回改为显式
  library_go_parent（访问放开，可越过 TF 根到 /）。lock 页继续自行消费返回。
- 打开册：缩略图源图改用预算解码（meow_img_load_file_dim，峰值数十 MB→~1MB）；
  离开书库页停止后台探测/缩略图线程，避免与 open_book 争内存/FD；打开失败按
  errno 记日志，ENOMEM/EMFILE/ENFILE/EAGAIN/EINTR 自动重试一次；修 open_book
  自拷贝 snprintf 的未定义行为。
- 测试：test_browse_ui_host 增加 TF 根退出/顶栏上级/左缘不拖拽断言；新增
  test_open_book_host（封面图+含图子目录的父目录 → reader 打开的完整 UI 链路）。

tuku/test/test_browse_ui_host.c

@@ -39,6 +39,8 @@ int         library_test_hint_active(void);
 const char *library_test_hint(void);
 int         library_test_go_root(void);
 int         library_test_go_up(void);
+int         library_test_go_parent(void);
+int         library_test_drag_active(void);
 int         library_test_has_list(void);
 const char *library_test_cache_dir(void);
 int         library_test_cache_on(void);
@@ -336,6 +338,30 @@ int main(void)
     CHECK(library_test_go_up() == 0, "go_up at / returns 0 (app exits)");
     CHECK(strcmp(library_test_dir(), "/") == 0, "still at / after exit gesture");
 
+    /* ---- U3b：系统返回语义 = TF 根退出 App；顶栏「上级」保留越界浏览 ---- */
+    CHECK(library_test_load(root) == 1, "load fixture root");
+    CHECK(library_test_go_up() == 0, "go_up at TF root returns 0 (app exits)");
+    CHECK(strcmp(library_test_dir(), root) == 0, "still at TF root after exit gesture");
+
+    /* 顶栏显式「上级」：访问放开，TF 根 → 其父目录（fixture root 的 parent = data）。 */
+    CHECK(library_test_go_parent() == 1, "explicit up from TF root consumed");
+    CHECK(strcmp(library_test_dir(), data) == 0,
+          "explicit up goes above TF root (%s)", library_test_dir());
+    CHECK(library_test_load(root) == 1, "back to fixture root");
+
+    /* ---- U3c：左缘 24px 系统返回手势带不启动列表拖拽 ---- */
+    int row_y = list_top + 28;
+    library_on_touch_down(120, row_y);
+    CHECK(library_test_drag_active() == 1, "touch inside list starts drag");
+    library_on_touch_down(5, row_y);
+    CHECK(library_test_drag_active() == 0, "left-edge touch does not start drag");
+    library_on_touch_down(23, row_y);
+    CHECK(library_test_drag_active() == 0, "left-edge (23px) does not start drag");
+    library_on_touch_down(24, row_y);
+    CHECK(library_test_drag_active() == 1, "just outside left band starts drag");
+    library_on_touch_down(120, meow_ui_height() - 4);
+    CHECK(library_test_drag_active() == 0, "bottom hotzone does not start drag");
+
     /* ---- U4：不可读目录 → 停留 + 提示 ---- */
     CHECK(library_test_load(root) == 1, "back to fixture root");
     char noread[MR_PATH_MAX];