engine: add the footage family to the C ABI facade; oak-cli probe

- oakengine_footage_probe inspects media without a project (decoder
  probe): stream counts, per-stream video info (dimensions, rate,
  duration, color tags, interlacing), audio info, duration, decoder
  name, source start time; thread_local last_error for the NULL-handle
  failure paths
- oakengine_project_import_footage adds probed footage to a project
  through the same undoable command path as the app
- oak-cli probe prints decoder/duration/per-stream details and runs as
  a ctest everywhere (no GL)
- dual ownership documented: probe handles are owned, imported footage
  is borrowed from its project

engine/CMakeLists.txt

@@ -264,4 +264,9 @@ if (BUILD_TESTS)
     if (TARGET olive-render-worker)
         add_dependencies(oakengine_renderer_test olive-render-worker)
     endif ()
+
+    make_oakengine_test(oakengine_footage_test)
+    target_compile_definitions(oakengine_footage_test PRIVATE
+            OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+    )
 endif ()