Usage accounting over a run's results — the documented convention hosts follow so cost lands per-run without this library knowing about pricing.

Steps that spend put a "usage" map in their result:

%{"input_tokens" => 120, "output_tokens" => 40, "calls" => 1}

Missing keys count as zero; results that are not maps, and maps without "usage", contribute nothing. Hosts price the folded totals themselves (the pricing oracle is a host concern, never this library's).