fix: M6.41批 — 启动失败原因透传 + 阅读器左缘返回/阅读页接入view栈 + 后台管理按钮点不动 + 阅读器瘦身(fonts common+页位图释放) + files txt换行

- launcher(453): app_launcher.c g_launch_fail_reason+launch_fail()统一出口(5失败点 fork OOM/exec/no such file/missing dep/execpipe)+main_menu.c launch_fail_toast(缺依赖/CANNOT START: name(reason))+toast256
- launcher(457): bg_page.c bgm_enumerate跳过term_at_ms(杀后当下消失)+不可杀行按钮置灰+row_tap/top_action返回1; subpage.c 消warning
- reader(454): reader_on_swipe LEFT/RIGHT→返回(先关overlay否则page_pop回书架)+app.c书架根LEFT→exit
- reader(458瘦身A+B): app.json fonts full→common(省13MB atlas)+reader.c RD_PAGE_CACHE 3→2+g_anim_old懒分配/anim_done释放+on_enter/on_leave页位图离开释放重建
- reader(459导航): reader_open返回int+shelf点书push ACT_READER(阅读页接入view栈,on_enter/on_leave驱动) — 使454左缘返回+458瘦身B真正生效
- files(455): viewer.c viewer_cp_len+按码位折行(txt中文不冲出右缘)

0warning;宿主测试全过;files二进制已热更

patch_v2/reader_app/app.c

@@ -29,7 +29,11 @@ static void app_on_swipe(int dir, int dx, int dy)
     (void)dy; (void)dx;
     if (settings_is_active()) { return; }        /* 设置页滑动不翻页 */
     if (reader_is_active()) { reader_on_swipe(dir, dx, dy); return; }
-    if (shelf_is_active()) { shelf_on_swipe(dir, dx, dy); return; }
+    if (shelf_is_active()) {
+        /* 书架根页左缘右滑（返回手势）→ 退出 App（镜像 hello_app/files_app 的 LEFT→返回）。 */
+        if (dir == MEOW_SWIPE_LEFT) { meow_ui_request_exit(); return; }
+        shelf_on_swipe(dir, dx, dy); return;
+    }
     if (dir == MEOW_SWIPE_LEFT) { meow_ui_request_exit(); }
 }