feat(updater): stage3 resilient gui update flow

- added retry-based file copy, rollback restart, and version marker validation in updater GUI

- added build step to write dist/version.txt for post-update validation

- added unit tests for updater helpers

tests/test_updater_gui.py

@@ -0,0 +1,38 @@
+import importlib.util
+import tempfile
+import unittest
+from pathlib import Path
+
+
+MODULE_PATH = Path("updater_gui.py")
+SPEC = importlib.util.spec_from_file_location("updater_gui_under_test", MODULE_PATH)
+updater_gui = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(updater_gui)
+
+
+class UpdaterGuiTests(unittest.TestCase):
+    def test_read_version_marker(self):
+        with tempfile.TemporaryDirectory() as tmp_dir:
+            marker = Path(tmp_dir) / "version.txt"
+            marker.write_text("2.0.1\n", encoding="utf-8")
+            value = updater_gui._read_version_marker(tmp_dir)
+            self.assertEqual(value, "2.0.1")
+
+    def test_mirror_tree_skips_selected_file(self):
+        with tempfile.TemporaryDirectory() as src_tmp, tempfile.TemporaryDirectory() as dst_tmp:
+            src = Path(src_tmp)
+            dst = Path(dst_tmp)
+            (src / "keep.txt").write_text("ok", encoding="utf-8")
+            (src / "skip.bin").write_text("x", encoding="utf-8")
+            (src / "sub").mkdir()
+            (src / "sub" / "nested.txt").write_text("nested", encoding="utf-8")
+
+            updater_gui._mirror_tree(str(src), str(dst), skip_names={"skip.bin"})
+
+            self.assertTrue((dst / "keep.txt").exists())
+            self.assertTrue((dst / "sub" / "nested.txt").exists())
+            self.assertFalse((dst / "skip.bin").exists())
+
+
+if __name__ == "__main__":
+    unittest.main()