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/disk.h

@@ -147,6 +147,49 @@ OAKENGINE_API void *oakengine_disk_get_open_folder(const char *path);
 OAKENGINE_API int oakengine_disk_invalidate_project(
 		OakEngineProject *project);
 
+/* ---- DiskCacheFolder accessors -------------------------------------------------
+ *
+ * Accessors for a borrowed folder handle from
+ * oakengine_disk_get_open_folder() (olive::DiskCacheFolder, passed as a
+ * plain `void *` like it is returned). The limit is a byte count carried
+ * as a double (the engine stores a qint64; a double is exact up to
+ * 2**53 bytes, far beyond any cache size).
+ */
+
+/**
+ * @brief Cache size limit of the folder in bytes
+ * (DiskCacheFolder::get_limit()). 0 on a NULL handle.
+ */
+OAKENGINE_API double oakengine_disk_folder_get_limit(const void *folder);
+
+/**
+ * @brief Set the cache size limit in bytes
+ * (DiskCacheFolder::set_limit()). `limit` < 0 yields
+ * OAKENGINE_E_INVALID.
+ */
+OAKENGINE_API int oakengine_disk_folder_set_limit(void *folder, double limit);
+
+/**
+ * @brief 1 if the folder is cleared when the application closes
+ * (DiskCacheFolder::get_clear_on_close()). 0 on a NULL handle.
+ */
+OAKENGINE_API int oakengine_disk_folder_get_clear_on_close(
+		const void *folder);
+
+/**
+ * @brief Set the clear-on-close flag (DiskCacheFolder::set_clear_on_close()).
+ * Returns OAKENGINE_OK or OAKENGINE_E_INVALID.
+ */
+OAKENGINE_API int oakengine_disk_folder_set_clear_on_close(void *folder,
+														   int clear);
+
+/**
+ * @brief The folder's path (DiskCacheFolder::get_path(); buf/size
+ * convention). Returns OAKENGINE_E_INVALID on a NULL handle.
+ */
+OAKENGINE_API int oakengine_disk_folder_get_path(const void *folder,
+												 char *buf, int buf_size);
+
 #ifdef __cplusplus
 }
 #endif