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/common/serializedlayoutinfoapp.h

@@ -0,0 +1,76 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_SERIALIZEDLAYOUTINFOAPP_H
+#define OAK_SERIALIZEDLAYOUTINFOAPP_H
+
+#include <map>
+#include <vector>
+
+#include <QByteArray>
+#include <QString>
+
+#include "oakengine/node.h"
+
+namespace olive
+{
+
+/**
+ * @brief App-side mirror of the engine's olive::SerializedLayoutInfo
+ * (engine/node/project/serializer/serializedlayoutinfo.h).
+ *
+ * SYNC OBLIGATION: the data member layout (types AND order) must stay
+ * identical to the engine type. Instances of this struct cross the engine
+ * boundary as `void *` (Core::save_project_internal() passes one to
+ * oakengine_task_create_project_save(), and the engine hands one back
+ * through the load_layout callback); the engine side reinterprets the
+ * pointer as its own olive::SerializedLayoutInfo and copies the members,
+ * so any layout divergence is silent memory corruption.
+ *
+ * The engine's std::vector<Folder*> / std::vector<Sequence*> /
+ * std::vector<ViewerOutput*> members are mirrored as
+ * std::vector<OakEngineNode*> (same pointer size and semantics: borrowed
+ * node handles). panel_data mirrors
+ * std::map<QString, PanelLayoutInfo> where PanelLayoutInfo is
+ * std::map<QString, QString> (identical to PanelWidget::Info).
+ *
+ * Only the data members are mirrored; the engine type's XML
+ * (de)serialization methods (to_xml/from_xml) stay engine-side.
+ */
+class SerializedLayoutInfo {
+public:
+	SerializedLayoutInfo() = default;
+
+	QByteArray state;
+
+	std::vector<OakEngineNode *> open_folders;
+
+	std::vector<OakEngineNode *> open_sequences;
+
+	std::vector<OakEngineNode *> open_viewers;
+
+	std::map<QString, std::map<QString, QString>> panel_data;
+};
+
+}
+
+Q_DECLARE_METATYPE(olive::SerializedLayoutInfo)
+
+#endif // OAK_SERIALIZEDLAYOUTINFOAPP_H