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)

worker/CMakeLists.txt

@@ -23,7 +23,6 @@
 # dependencies is a later-phase cleanup (render-process-isolation plan).
 add_executable(olive-render-worker
         workermain.cpp
-        $<TARGET_OBJECTS:libolive-editor>
         $<TARGET_OBJECTS:olive-version-obj>
 )
 set_target_properties(olive-render-worker PROPERTIES OUTPUT_NAME "oak-render-worker")
@@ -75,6 +74,7 @@ if (WIN32)
     add_custom_command(TARGET olive-render-worker POST_BUILD
             COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:ffmpeg_bridge> $<TARGET_FILE_DIR:olive-render-worker>
             COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:olivecore> $<TARGET_FILE_DIR:olive-render-worker>
+            COMMAND ${CMAKE_COMMAND} -E copy_if_different $<TARGET_FILE:oakengine> $<TARGET_FILE_DIR:olive-render-worker>
     )
 endif ()