R8: finish app/ pure C ABI migration (P3-P9) and make OTIO required

- app/ no longer includes engine C++ headers nor holds engine C++ types:
  engine access goes through the oakengine C ABI plus C++ wrappers
  (oakutil/oaknode.h, oakutil/oakvideo.h) and app-local mirror types
  (tooltypes, trackreferencehandle, timelinecommonapp, keyframetypes,
  subtitleapp, serializedlayoutinfoapp, nodevaluehandle, sliderdisplaytypeapp)
- engine: new C ABI functions for block/track/clip/transition navigation
  and predicates, links, caches, waveform/playback, disk folder,
  sequence_track_list, node_free, footage_is_valid, block_get_track,
  get_brush; loadotio/saveotio ported to the current engine API
- OTIO is now a required dependency: CI and CD build it on every
  platform, FindOpenTimelineIO fixed for OTIO 0.16/0.19 (the old deps
  include requirement silently disabled OTIO everywhere), runtime
  libraries are bundled into packages and copied next to macOS binaries
  (oak_copy_otio_runtime)
- fix ProjectViewModel drag&drop mime read/write size mismatch (segfault)
- unify color label naming (k_olive -> "Oak") in the app-side mirror
- docs: OTIO required, FFmpeg minimum corrected to 6.0 (en/zh)
- gtest suite: 1925 passed, 0 failed

tests/gtest/node_view_test.cpp

@@ -8,6 +8,17 @@
 
 using namespace olive;
 
+namespace
+{
+
+// Helper: wrap an engine Node* as oak::Node for the C ABI widget interface
+inline oak::Node to_oak(olive::Node *n)
+{
+	return oak::Node(reinterpret_cast<OakEngineNode *>(n));
+}
+
+} // namespace
+
 class NodeViewTest : public ::testing::Test {
 protected:
 	void SetUp() override
@@ -34,10 +45,10 @@ TEST_F(NodeViewTest, SetContextsUpdatesContextList)
 	solid->setParent(project_.get());
 
 	NodeView view;
-	view.set_contexts({ solid });
+	view.set_contexts({ to_oak(solid) });
 
 	EXPECT_EQ(view.get_contexts().size(), 1);
-	EXPECT_EQ(view.get_contexts().first(), solid);
+	EXPECT_EQ(view.get_contexts().first(), to_oak(solid));
 }
 
 TEST_F(NodeViewTest, ShowSelectedNodeInParamEditorNoSelectionIsNoOp)
@@ -71,7 +82,7 @@ TEST_F(NodeViewTest, ClearGraphRemovesContexts)
 	solid->setParent(project_.get());
 
 	NodeView view;
-	view.set_contexts({ solid });
+	view.set_contexts({ to_oak(solid) });
 	EXPECT_FALSE(view.get_contexts().isEmpty());
 
 	view.clear_graph();