ci: link render backend plugins against oakengine-obj on Windows; switch runners

- liboakengine.dll does not export engine-internal C++ symbols
  (olive::Renderer, FileFunctions, VideoParams, Texture), so oakgl.dll
  failed to link on Windows. Embed oakengine-obj instead (same pattern
  as the engine tests); MinGW's name-based RTTI keeps cross-DLL casts
  working.
- ci.yml: move to warp runners

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04, macos-15, windows-latest]
+        os: [warp-ubuntu-latest-x64-16x, warp-macos-15-arm64-12x, warp-windows-latest-x64-32x]
     env:
       CMAKE_BUILD_TYPE: Release
       CCACHE_DIR: ${{ github.workspace }}/.ccache