build: move render worker out of app/ into top-level worker/

- oak-render-worker now builds from worker/ (own CMakeLists.txt) as a
  peer of app/; RenderWorkerPool resolves the new build-tree location
- deduplicated the Linux install() rules for the worker
- worker-spawning tests resolve build/worker instead of build/app
- cd.yml: Windows staging copies the worker from its new output path

tests/gtest/render_direct_connection_test.cpp

@@ -56,11 +56,11 @@ QString demo_video_path()
 QString worker_binary_path()
 {
 	// The test binary lives in cmake-build-debug/tests/gtest; the worker is in
-	// cmake-build-debug/app.
+	// cmake-build-debug/worker.
 	QDir dir(QCoreApplication::applicationDirPath());
 	dir.cdUp(); // tests/gtest -> tests
 	dir.cdUp(); // tests -> build dir
-	dir.cd(QStringLiteral("app"));
+	dir.cd(QStringLiteral("worker"));
 #if defined(_WIN32)
 	return dir.filePath(QStringLiteral("oak-render-worker.exe"));
 #else