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

engine/include/oakengine/project.h

@@ -226,6 +226,21 @@ OAKENGINE_API int oakengine_folder_has_child_recursive(
 OAKENGINE_API int oakengine_folder_index_of_child(
 	const OakEngineNode *folder, const OakEngineNode *child);
 
+/**
+ * @brief Number of direct item children of a folder
+ * (Folder::item_child_count()). 0 when `folder` is NULL or not a Folder.
+ */
+OAKENGINE_API int oakengine_folder_item_child_count(
+	const OakEngineNode *folder);
+
+/**
+ * @brief Borrowed handle of the item child at `index`
+ * (Folder::item_child()). NULL when out of range or `folder` is not a
+ * Folder.
+ */
+OAKENGINE_API OakEngineNode *oakengine_folder_item_child(
+	const OakEngineNode *folder, int index);
+
 /**
  * @brief Static input key string for Folder children (Folder::k_child_input).
  * Never freed.