engine: add the keyframe family to the node facade

- keyframed state, count, read (time in sequence timebase + mapped
  values), add/remove, easing read/write (linear/bezier/hold with
  control points), and clear - all undoable with full undo/redo
  assertions
- easing set commands are minimal capi-local UndoCommands matching
  the app-layer semantics (engine has none of its own); same-time
  duplicates are rejected with E_STATE instead of hitting the engine's
  debug assert

engine/CMakeLists.txt

@@ -306,4 +306,9 @@ if (BUILD_TESTS)
     target_compile_definitions(oakengine_node_test PRIVATE
             OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
     )
+
+    make_oakengine_test(oakengine_keyframe_test)
+    target_compile_definitions(oakengine_keyframe_test PRIVATE
+            OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+    )
 endif ()