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/shared/ipc.ts

@@ -35,6 +35,13 @@ export const Ipc = {
   SESSION_OPEN: 'session:open',
   /** main keeps a short replay buffer per session so late subscribers catch up */
   SESSION_REPLAY: 'session:replay',
+  /**
+   * Compensating query for a missed PTY_EXIT: that broadcast fires exactly once
+   * and is never replayed, so a pane bound to a shell that died before it
+   * subscribed would wait forever. Distinct from SESSION_STATE_GET/SET, which
+   * carry the layout snapshot.
+   */
+  SESSION_STATE: 'session:state',
 
   // ---- ssh connections (bookmarks) ----
   CONNECTIONS_LIST: 'connections:list',
@@ -216,6 +223,16 @@ export interface PtyExitEvent {
   exitCode: number
 }
 
+/** Answer to SESSION_STATE: what the main process still knows about a session. */
+export interface SessionStateResult {
+  /** the session is alive and still accepts data */
+  exists: boolean
+  /** the session exited; `exitCode` then carries the status it died with */
+  exited: boolean
+  /** null unless `exited` */
+  exitCode: number | null
+}
+
 export interface AppInfo {
   platform: NodeJS.Platform | string
   appVersion: string