feat(flow): on-demand host handoff with reliable return and a back-tap tip

Keyboard
- Only jump to the app when the voice surface is shown (tab tap / opened
  there); typing tabs no longer leave the host on keyboard open.
- Wait up to 0.4 s for the arbiter to report the host app before leaving,
  and late-bind an id that arrives within 6 s of the jump.
- Empty arbiter callbacks no longer wipe an already captured host id;
  install the capture handler in every build configuration.

App
- Return to the previous app with up to four retries using iOS's real
  open result, and honor the skip-app-switch preference.
- Keep the pending host id across cold-start zombie reconciliation, and
  expire it after two minutes (legacy values without a timestamp stay valid).
- Show a light tip pointing at the system back control when we cannot
  return automatically ("点击返回，立即使用").

Tests: TTL behaviour and return retry policy.

OSGKeyboardExt/Utilities/OSGHostArbiterProbe.m

@@ -44,8 +44,12 @@ static void OSGSwizzledKeyboardChanged(id self, SEL _cmd, id change, id completi
     NSString *detail = change ? NSStringFromClass([change class]) : @"change=nil";
 
     os_unfair_lock_lock(&sLock);
-    // 读不到 ID 的回调同样覆盖旧值：宿主已经变了，旧 ID 不能再用。
-    sLastCapturedBundleId = bundleId.length > 0 ? bundleId : nil;
+    // 只在读到有效 ID 时更新。读不到 ID 的回调（焦点切换、空事件）不再清掉已拿到的值：
+    // 之前“最后一个事件为准”会让一次空回调把正确的宿主抹掉，是识别忽有忽无的来源之一。
+    // 宿主真的变了，由键盘收起时的 invalidateCapturedBundleId 作废旧值。
+    if (bundleId.length > 0) {
+        sLastCapturedBundleId = bundleId;
+    }
     OSGHostArbiterCaptureHandler handler = sHandler;
     os_unfair_lock_unlock(&sLock);
     if (handler) {