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

app/widget/timebased/timebasedview.h

@@ -72,12 +72,12 @@ public:
 	{
 	}
 
-	ViewerOutput *get_viewer_node() const
+	OakEngineNode *get_viewer_node() const
 	{
 		return viewer_;
 	}
 
-	void set_viewer_node(ViewerOutput *v);
+	void set_viewer_node(OakEngineNode *v);
 
 	QPointF scale_point(const QPointF &p) const;
 	QPointF unscale_point(const QPointF &p) const;
@@ -146,7 +146,7 @@ private:
 
 	double y_scale_;
 
-	ViewerOutput *viewer_;
+	OakEngineNode *viewer_;
 
 	int64_t viewer_sub_ = 0;
 };