The Heun corrector evaluates the model at sigma_next, and the terminal step
lands on sigma 0 — degenerate, and it returns early before the corrector is
reached. The window was measured back from the end of the schedule, so it
started on that step: final_correction_steps=1, the default, performed zero
corrections on any schedule ending at 0.
Counted over correctable steps instead, so N means N.
Default moves 1 -> 0: fixing the arithmetic alone would have added a model
call to every existing run (~14% on a 7-step schedule). 0 is what every run
has actually been doing. quality_sharpness shares the window by design.
