engine: timeline edit primitives and oak-cli transcode

- oakengine_sequence_add_track and add_footage_clip are the facade's
  first editing primitives: undoable track creation and clip placement
  with full range validation, clip enumeration, and gap filtering
- oak-cli transcode closes the loop: media file -> import -> clip ->
  render, producing scaled PPM frames and a WAV from just the C ABI
- engine fix uncovered by transcode: the render worker used an invalid
  empty AudioParams for IPC render frames, crashing any sequence that
  contains audio; it now derives them from the rendered node itself
- sequence_new hardens its defaults against missing audio config keys

engine/CMakeLists.txt

@@ -269,4 +269,9 @@ if (BUILD_TESTS)
     target_compile_definitions(oakengine_footage_test PRIVATE
             OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
     )
+
+    make_oakengine_test(oakengine_timeline_edit_test)
+    target_compile_definitions(oakengine_timeline_edit_test PRIVATE
+            OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+    )
 endif ()