fix(main): PTY_EXIT 补偿查询、SSH 飞行连接登记与双窗口守卫

- 新增 SESSION_STATE 查询通道（契约先行）：PTY_EXIT 只广播一次不重放，
  绑定晚于退出的窗格订阅后补查一次退出状态，快速退出的 shell 不再留下
  永远空白、无死亡遮罩的窗格（退出码登记表上限 512 条 FIFO）
- openSession 在 connectSsh 返回前登记 pendingOpens（owner + onClient
  最早引用），killPtysByOwner/killAllPtys 可中止飞行中的握手，renderer
  崩溃后不再留下无主孤儿 SSH 会话
- whenReady 的 createWindow 加已有窗口守卫，堵住与 second-instance
  showOrCreate 的双窗口竞态
- killAllPtys 补 replayBuffers.clear()，与 killSession 拆卸清单对齐
- promptUser 注释 30s 改为实际 120s；pty.ts 顺带注入 zmodem 背压钩子
  （见下一提交）

src/main/index.ts

@@ -100,7 +100,10 @@ if (!gotSingleInstanceLock) {
       // so the menu teardown is applied here from the flag itself.
       const lock = initLockController()
       applyMenuLockState(lock.isLocked())
-      createWindow()
+      // A second launch that raced this startup already created (or surfaced) a
+      // window from its `second-instance` handler; creating another here would
+      // leave two. Same guard the activate handler below uses.
+      if (BrowserWindow.getAllWindows().length === 0) createWindow()
       initTray(showOrCreate)
       // Tray labels are resolved from the dictionary at build time, so the menu has
       // to be rebuilt whenever the interface language changes.