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/ipc.ts

@@ -34,7 +34,7 @@ import {
 import { broadcast } from './broadcast'
 import { CommandsStore, defaultCommandsPath } from './commands'
 import type { CommandItem } from '../shared/commands'
-import { createPty, killPty, resizePty, writePty, openSession, configureSessionRuntime, getSessionReplay, registerLogHooks } from './pty'
+import { createPty, killPty, resizePty, writePty, openSession, configureSessionRuntime, getSessionReplay, sessionState, registerLogHooks } from './pty'
 import { respondZmodem } from './zmodem'
 import type { ZmodemResponse } from '../shared/ipc'
 
@@ -153,6 +153,9 @@ export function registerIpc(): void {
   ipcMain.handle(Ipc.PTY_CREATE, (event, opts?: PtyCreateOptions) => createPty(opts, event.sender.id))
   ipcMain.handle(Ipc.SESSION_OPEN, (event, opts: SessionOpenOptions) => openSession(opts, event.sender.id))
   ipcMain.handle(Ipc.SESSION_REPLAY, (_event, id: string) => getSessionReplay(id))
+  // Compensates the one-shot PTY_EXIT broadcast: a pane that subscribed after
+  // its shell died asks here instead of waiting forever.
+  ipcMain.handle(Ipc.SESSION_STATE, (_event, id: string) => sessionState(id))
   ipcMain.on(Ipc.PTY_WRITE, (_event, id: string, data: string) => writePty(id, data))
   ipcMain.on(Ipc.PTY_RESIZE, (_event, id: string, cols: number, rows: number) =>
     resizePty(id, cols, rows)