The three shipped copies of ai_engine.h (proj_cm55/modules/ai_models/, lib/edge_ai/include/, dist/edge_ai/include/) are byte-identical.
The source-tree header additionally declares ai_engine_register(), ai_engine_dyn_count() and ai_engine_dyn_capacity(). They are not in api.txt and not callable in the shipped archive.
ai_result_t has no valid, label or confidence field. The shipped proj_cm55/modules/ai_models/README.md:33-40 example that uses them does not compile; the real reduction is the inline ai_result_top_positive() (ai_engine.h:249-265), which returns the strongest positive class (index 0 is the negative class by registry contract) as 0..100.
ai_engine_stack_words() returning 0 means the inference task was never created; every reading on every page below is vacuous until it is non-zero.