Idle control on ETB using throttle flow model

Right now, it goes idle % -> ETB position, linearly mapped.

Could instead map idle % to a range of mass air flow rate (either 0=0, 100%=max g/s, or map g/s to both min/max so you can't go to "true zero"). Then use the throttle flow model to convert target airflow -> throttle position.

Start by tracing where idle percentage is mapped to ETB position and where the throttle flow model converts airflow targets to throttle position. Compare the current linear mapping with the proposed mass-airflow range, then verify that the selected mapping produces the intended idle airflow without allowing unintended true-zero airflow.

Written by the indexing model from the issue text.