test: add MainWindow structure smoke test (skips headless)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

conftest.py

@@ -1,2 +1,6 @@
 import sys, os
 sys.path.insert(0, os.path.dirname(__file__))
+
+
+def pytest_configure(config):
+    config.addinivalue_line("markers", "gui: constructs Qt widgets; needs a display")

tests/test_ui_structure.py

@@ -0,0 +1,35 @@
+import os
+import pytest
+
+# A real platform is needed because MpvWidget creates a GL context.
+# If construction fails for any environment reason, skip — this test is a
+# best-effort structural net, not a gate on core/ tests.
+pytestmark = pytest.mark.gui
+
+
+@pytest.fixture(scope="module")
+def app():
+    from PyQt6.QtWidgets import QApplication
+    inst = QApplication.instance() or QApplication([])
+    yield inst
+
+
+@pytest.fixture
+def win(app):
+    try:
+        from main import MainWindow
+        w = MainWindow()
+    except Exception as e:  # GL/mpv/display unavailable, etc.
+        pytest.skip(f"MainWindow could not be constructed here: {e}")
+    yield w
+    w.close()
+    w.deleteLater()
+
+
+def _descendant_object_names(widget):
+    """All objectNames in a widget's child tree (for containment asserts)."""
+    return {c.objectName() for c in widget.findChildren(object) if c.objectName()}
+
+
+def test_window_constructs(win):
+    assert win.windowTitle() == "8-cut"