Add parameter to adjust where SimTrackerHits are placed in APV25 window

[bloodyyugo]

For pulser MC, the signal tracks come close to the edge of where the accidental population falls off. See:

https://indico.slac.stanford.edu/event/9927/contributions/13173/attachments/5777/14765/hps-mc-timeranges-aug12-2025.pdf

...we should put in a settable parameter in the SvtDigi code to allow us to move this around.