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/window/mainwindow/mainwindowundo.cpp

@@ -31,7 +31,7 @@ namespace olive
 namespace {
 
 struct OpenCloseSequenceData {
-	Sequence *sequence;
+	OakEngineNode *sequence;
 };
 
 void open_sequence_redo(void *userdata)
@@ -65,19 +65,21 @@ void open_close_sequence_free(void *userdata)
 
 } // anonymous namespace
 
-void *make_open_sequence_command(Sequence *sequence)
+void *make_open_sequence_command(oak::Node sequence)
 {
 	auto *d = new OpenCloseSequenceData;
-	d->sequence = sequence;
+	d->sequence = sequence.handle();
+	// WRAPPER-GAP: oakengine_undo_command_create (undo command assembly)
 	return oakengine_undo_command_create(nullptr, open_sequence_redo,
 										 open_sequence_undo,
 										 open_close_sequence_free, d);
 }
 
-void *make_close_sequence_command(Sequence *sequence)
+void *make_close_sequence_command(oak::Node sequence)
 {
 	auto *d = new OpenCloseSequenceData;
-	d->sequence = sequence;
+	d->sequence = sequence.handle();
+	// WRAPPER-GAP: oakengine_undo_command_create (undo command assembly)
 	return oakengine_undo_command_create(nullptr, close_sequence_redo,
 										 close_sequence_undo,
 										 open_close_sequence_free, d);