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)

.github/workflows/cd.yml

@@ -69,10 +69,12 @@ jobs:
           cp build/ffmpeg_bridge/bin/ffmpeg_bridge.dll app/packaging/windows/nsis/oak-editor/
           # Same for the liboakcore DLL
           cp build/core/oakcore.dll app/packaging/windows/nsis/oak-editor/
+          # Same for the liboakengine DLL
+          cp build/engine/oakengine.dll app/packaging/windows/nsis/oak-editor/
           windeployqt6 app/packaging/windows/nsis/oak-editor/oak-editor.exe
           # Copy all non-Qt MSYS2 DLLs recursively for every binary we ship
           cd app/packaging/windows/nsis/oak-editor
-          for binary in oak-editor.exe oak-render-worker.exe oakgl.dll oakvulkan.dll ffmpeg_bridge.dll oakcore.dll; do
+          for binary in oak-editor.exe oak-render-worker.exe oakgl.dll oakvulkan.dll ffmpeg_bridge.dll oakcore.dll oakengine.dll; do
             [ -f "$binary" ] || continue
             for l in $(ntldd -R "$binary" | grep -E 'mingw64|ucrt64|clang64' | sed 's/^[ \t]*//' | cut -d' ' -f3); do
               cp -v "$l" .
@@ -372,10 +374,12 @@ jobs:
           ls AppDir/usr/lib/libvulkan.so*                    || (echo "Missing libvulkan" && exit 1)
           ls AppDir/usr/lib/libffmpeg_bridge.so              || (echo "Missing libffmpeg_bridge.so" && exit 1)
           ls AppDir/usr/lib/liboakcore.so                    || (echo "Missing liboakcore.so" && exit 1)
+          ls AppDir/usr/lib/liboakengine.so                  || (echo "Missing liboakengine.so" && exit 1)
           # Ensure every shipped binary resolves all of its dependencies
           ! ldd AppDir/usr/bin/oak-editor | grep "not found"
           ! ldd AppDir/usr/bin/oak-render-worker | grep "not found"
           ! ldd AppDir/usr/lib/liboakcore.so | grep "not found"
+          ! ldd AppDir/usr/lib/liboakengine.so | grep "not found"
           ! ldd AppDir/usr/lib/liboakgl.so | grep "not found"
           ! ldd AppDir/usr/lib/liboakvulkan.so | grep "not found"