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

CMakeLists.txt

@@ -309,6 +309,7 @@ add_subdirectory(third_party/KDDockWidgets EXCLUDE_FROM_ALL)
 
 add_subdirectory(third_party/openfx/HostSupport)
 add_subdirectory(app)
+add_subdirectory(worker)
 
 if (BUILD_TESTS)
   enable_testing()