feat: M6.43 — reader分段流式读取(Phase1,只txt) + 面板前台管理/返回回桌面按钮

- reader分段(只txt): core/seg.{h,c} 512KB码位对齐切段(尾字完整)+manifest+按需读单段; book.c rd_book_open只布局每页所在段(治CPU100%/返回无效) load_seg只读当前段(非txt原路径); reader.c 进度实时保存(翻页/刮擦/字号/编码dirty+on_tick节流落盘+退出即时写,锚点存段号,崩溃不丢进度); rdtxt.c fread替代fgetc(去开书扫描卡顿); Phase2(双段拼接+4%释放)留待
- 面板: /api/ams/bg加fg/fg_count(后台保持向后兼容)+POST /api/ams/:pkg/killfg(前台救命:优先am kill,launcher拒则pid直信号SIGTERM->SIGKILL绕过门控)+前端/#/bg前台组'终止'绿徽标+确认弹窗(一键清仍只后台不误杀前台); POST /api/input/gesture(返回=左缘右滑/回桌面=底部上滑,注入滑动轨迹复用SDK手势)+前端屏幕控制页加返回/回桌面MD3按钮

0warning;宿主test_reader_core 72-0/seg_smoke 19-0/loaders_smoke/book_smoke/formats_smoke PASS;面板selftest/node check PASS

patch_v2/meowpanel/web/index.html

@@ -69,7 +69,7 @@
         <span class="nav-ico"><svg viewBox="0 0 24 24"><path d="M4 6h16M4 12h16M4 18h16" stroke="currentColor" stroke-width="2" stroke-linecap="round"/><circle cx="9" cy="6" r="2.2" fill="var(--bg)"/><circle cx="15" cy="12" r="2.2" fill="var(--bg)"/><circle cx="7" cy="18" r="2.2" fill="var(--bg)"/></svg></span>
         <span class="nav-label">进程·服务</span>
       </button>
-      <button class="nav-item" data-route="bg"      title="后台活动（App 后台常驻：CPU/内存占用 + 一键清后台）">
+      <button class="nav-item" data-route="bg"      title="App 活动（前台可终止救命 · 后台常驻：CPU/内存占用 + 一键清后台）">
         <span class="nav-ico"><svg viewBox="0 0 24 24"><path d="M12 3 3 8l9 5 9-5-9-5z" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/><path d="M3 12l9 5 9-5" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/><path d="M3 16l9 5 9-5" fill="none" stroke="currentColor" stroke-width="2" stroke-linejoin="round"/></svg></span>
         <span class="nav-label">后台</span>
       </button>