test(updater): 覆盖调度、自动下载、同版本抑制与气泡

updater-fallback 从 82 扩到 150 断言（新增 11 节）：周期节拍与关闭、重排、
启动检查只武装一次、并发检查共用同一 promise、自动下载开关、同版本去重、
换版本恢复、手动检查不被抑制、已下载状态保持（含不卡 checking 与
fetchHandled/downloaded 区分）、气泡的窗口可见性守卫。electron-stub 的
Tray 补 displayBalloon 记录、BrowserWindow.getAllWindows 可注入假窗口。

AGENTS.md 更新机制一节补录三条来源、注入式调度、自动下载、同版本抑制与
气泡守卫。

tests/electron-stub.cjs

@@ -46,7 +46,10 @@ const fakeSession = (name) => {
 
 module.exports = {
   BrowserWindow: {
-    getAllWindows: () => []
+    // Configurable so a test can drive the "is a window visible?" guards (the
+    // update balloons, the session snapshot). Like the rest of the stub state
+    // this lives on the shared global, and the default is "no window open".
+    getAllWindows: () => state.windows ?? []
   },
   app: {
     getPath: (name) => {
@@ -110,6 +113,16 @@ module.exports = {
     setContextMenu() {}
     on() {}
     destroy() {}
+    /**
+     * Balloons (`notifyUpdate`, the minimize hint) have no other observable
+     * effect, so they are recorded instead of dropped. The sink is a global for
+     * the same reason as the rest of the state: the bundle inlines this file, so
+     * a test's require of this path is a different module instance.
+     */
+    displayBalloon(options) {
+      const sink = (globalThis.__otBalloons ??= [])
+      sink.push(options)
+    }
     static getBounds() {
       return { x: 0, y: 0, width: 0, height: 0 }
     }
@@ -123,6 +136,10 @@ module.exports = {
   __setPackaged: (v) => {
     state.isPackaged = v
   },
+  /** Fake window list `BrowserWindow.getAllWindows()` hands the bundled code. */
+  __setWindows: (wins) => {
+    state.windows = wins
+  },
   /** channel -> listener (invoke), `on:<channel>` -> listener (send). */
   get __handlers() {
     return state.handlers