加快门禁：删除测试窗口、释放桩线程并并行运行三个阶段

关闭后未删除的测试窗口让每次主题切换重排全部残留控件，ui_check 每次关窗还要等桩线程 5 秒。Linux 上结论不变，test_ui_smoke 75→64 s、ui_check 52→16 s；Windows 门禁结果见 .scratch/gate-speed/。

apps/studio/tests/conftest.py

@@ -97,6 +97,8 @@ def make_window(qt_app, tmp_path, monkeypatch):
 
     yield _make
 
+    from PySide6.QtCore import QCoreApplication, QEvent
+
     for window in created:
         try:
             window.close()
@@ -104,6 +106,14 @@ def make_window(qt_app, tmp_path, monkeypatch):
             # Reported rather than swallowed: a window that cannot close is a Qt lifetime
             # problem worth seeing, but it must not turn a green test red.
             logging.getLogger(__name__).debug("could not close a test window", exc_info=True)
+        # ``close()`` only hides a window: something in the window keeps it referenced,
+        # so it stayed alive for the rest of the session. Every leaked window's ~600
+        # widgets are then re-polished by each later ``app.setStyleSheet`` — measured on
+        # Linux, one theme switch takes 0.07 s with one window and 0.85 s after ten closed
+        # ones, and three theme tests took 76 s of a 231 s Windows suite. Delete it.
+        window.deleteLater()
+    created.clear()
+    QCoreApplication.sendPostedEvents(None, QEvent.Type.DeferredDelete)
 
 
 @pytest.fixture