[bls337]

• The stage's start position, speed and acceleration are kept in a StageScanState that stays null until a stage scan reads them, and finish() restores the stage only when it is set.
• A run refused before it reads the stage no longer writes a placeholder speed and acceleration to it, and a failed speed or acceleration read now refuses the run.
• PLogicScape calls StageScanUtils.computeScanSpeed directly, and its one-line computeScanSpeed wrapper is removed.