build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

CMakeLists.txt

@@ -308,6 +308,11 @@ set(KDDockWidgets_FRONTENDS "qtwidgets" CACHE INTERNAL "Only build the QtWidgets
 add_subdirectory(third_party/KDDockWidgets EXCLUDE_FROM_ALL)
 
 add_subdirectory(third_party/openfx/HostSupport)
+add_subdirectory(engine)
+
+# Everything above the engine library links against it
+list(APPEND OLIVE_LIBRARIES oakengine)
+
 add_subdirectory(app)
 add_subdirectory(worker)