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

cli/CMakeLists.txt

@@ -81,6 +81,15 @@ if (BUILD_TESTS)
     set_tests_properties(oak_cli_render PROPERTIES
             SKIP_RETURN_CODE 2
     )
+
+    # probe is headless and always runs; the output must mention the demo
+    # file's 1920x1080 video and 48000 Hz audio.
+    add_test(NAME oak_cli_probe
+            COMMAND oak-cli probe ${CMAKE_SOURCE_DIR}/tests/demo.mp4
+    )
+    set_tests_properties(oak_cli_probe PROPERTIES
+            PASS_REGULAR_EXPRESSION "1920.*48000"
+    )
     # Rendering needs the render worker and the dynamic backend plugins.
     if (TARGET olive-render-worker)
         add_dependencies(oak-cli olive-render-worker)