ci: fix three platform-specific build failures

- Linux: Qt6 metatype static_assert requires complete types; declare
  OakEngineBlock with Q_DECLARE_OPAQUE_POINTER in the public header
  (outside extern "C")
- Windows: gtest discovery runs the freshly linked test exe at build
  time and it cannot find the DLLs (0xc0000135); copy oakcore/oakengine/
  ffmpeg_bridge DLLs next to the test binaries on WIN32
- macOS: Imath header mixing between otio-install's stale bundled Imath
  and Homebrew's Imath broke ImathBox.h (V2h undeclared); bump the OTIO
  cache to v2 and drop bundled Imath headers so the system one is the
  only set in play

engine/CMakeLists.txt

@@ -292,6 +292,17 @@ if (BUILD_TESTS)
         # Freshly linked binaries can exceed the 5s default discovery
         # timeout on first run (dyld cold cache + Qt/OCIO init)
         gtest_discover_tests(${name} DISCOVERY_TIMEOUT 60)
+        if (WIN32)
+            # gtest discovery executes the binary at build time; it needs
+            # the engine DLLs next to it
+            add_custom_command(TARGET ${name} POST_BUILD
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                        $<TARGET_FILE:oakengine> $<TARGET_FILE_DIR:${name}>
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                        $<TARGET_FILE:olivecore> $<TARGET_FILE_DIR:${name}>
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                        $<TARGET_FILE:ffmpeg_bridge> $<TARGET_FILE_DIR:${name}>)
+        endif ()
     endfunction()
 
     make_oakengine_gtest(oakengine_ipc_test)

engine/include/oakengine/timeline.h

@@ -1308,6 +1308,11 @@ oakengine_clip_get_connected_viewer(const OakEngineBlock *clip);
 
 #ifdef __cplusplus
 }
+
+// Qt6 on some toolchains requires complete types in meta-type instantiations
+// (e.g. QList<OakEngineBlock*> in TimelinePanel signals); declare the handle
+// opaque instead of pulling the engine definition into the public header.
+Q_DECLARE_OPAQUE_POINTER(OakEngineBlock *)
 #endif
 
 #endif /* OAKENGINE_TIMELINE_H */