Function report_fill_stall 

fn report_fill_stall(supervisor: &mut SessionPixSupervisor)

Counts a fill stall, if one just began.

The planner cannot do this itself — it is a pure function of its inputs, and a metric is neither — so the edge is latched there and drained here, in the actor that is allowed to have effects. Not folded into dispatch, because the stall is not an action: it produces no work for the driver, only a number for the operator.