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

cmake/FindOpenTimelineIO.cmake

@@ -79,7 +79,9 @@ find_path(OTIO_DEPS_INCLUDE_DIR
         "OpenTimelineIO headers path"
 )
 
-list(APPEND OTIO_INCLUDE_DIRS ${OTIO_DEPS_INCLUDE_DIR})
+if(OTIO_DEPS_INCLUDE_DIR)
+    list(APPEND OTIO_INCLUDE_DIRS ${OTIO_DEPS_INCLUDE_DIR})
+endif()
 
 find_path(OT_INCLUDE_DIR
         opentime/rationalTime.h
@@ -123,11 +125,19 @@ find_library(OT_LIBRARY
 
 list(APPEND OTIO_LIBRARIES ${OT_LIBRARY})
 
+# OTIO_LIBRARY_DIR is only probed for .so above (never matches on macOS);
+# derive it from the located library as a fallback.
+if(NOT OTIO_LIBRARY_DIR AND OTIO_LIBRARY)
+    get_filename_component(OTIO_LIBRARY_DIR "${OTIO_LIBRARY}" DIRECTORY)
+endif()
+
 include(FindPackageHandleStandardArgs)
 
+# OTIO_DEPS_INCLUDE_DIR (bundled any/optional headers) only exists in OTIO
+# <= 0.15; v0.16 uses C++17 std::any and ships no deps headers, so it is
+# intentionally NOT part of REQUIRED_VARS.
 find_package_handle_standard_args(OpenTimelineIO
     REQUIRED_VARS
         OTIO_LIBRARIES
         OTIO_INCLUDE_DIRS
-        OTIO_DEPS_INCLUDE_DIR
 )