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/timebased_widget_test.cpp

@@ -15,8 +15,8 @@ TEST(TimeBasedWidget, ConnectedNodeClearsOnDelete)
 {
 	olive::TimeBasedWidget widget(false, false);
 	auto *viewer = new olive::ViewerOutput();
-	widget.connect_viewer_node(viewer);
-	EXPECT_EQ(widget.get_connected_node(), viewer);
+	widget.connect_viewer_node(reinterpret_cast<OakEngineNode *>(viewer));
+	EXPECT_EQ(widget.get_connected_node(), reinterpret_cast<OakEngineNode *>(viewer));
 	delete viewer;
 	EXPECT_EQ(widget.get_connected_node(), nullptr);
 }
@@ -63,11 +63,11 @@ TEST(SeekableWidget, SetMarkersAndWorkAreaAreReflected)
 	olive::TimelineMarkerList markers;
 	olive::TimelineWorkArea workarea;
 
-	widget.set_markers(&markers);
-	widget.set_work_area(&workarea);
+	widget.set_markers(reinterpret_cast<OakEngineMarkerList *>(&markers));
+	widget.set_work_area(reinterpret_cast<OakEngineWorkarea *>(&workarea));
 
-	EXPECT_EQ(widget.get_markers(), &markers);
-	EXPECT_EQ(widget.get_work_area(), &workarea);
+	EXPECT_EQ(widget.get_markers(), reinterpret_cast<OakEngineMarkerList *>(&markers));
+	EXPECT_EQ(widget.get_work_area(), reinterpret_cast<OakEngineWorkarea *>(&workarea));
 }
 
 TEST(SeekableWidget, MarkerEditingEnabledToggles)