Add optical step gather action

[Rashika-Gupta]

Partial implementation of step 1 from #1746, adding the infrastructure to gather optical step data into aux state during simulation.
Adds:

• OpticalStepStateData : state collection holding per-track slot, position, and volume ID for pre/post step points
• OpticalStepParams : AuxParams implementation registered with the aux registry,
• OpticalStepGatherExecutor<StepPoint P> : per-track functor that copies data from CoreTrackView into the step state; currently writes JSON output to optical_steps.jsonl as a debugging aid
• OpticalStepGatherAction : OpticalStepActionInterface that launches pre- and post-step executors; registered in CoreParams when CELER_OPTICAL_STEP env var is set

Known issues / cleanup needed before ready:

• OpticalStepInterface.hh is unused and should be removed
• Several commented-out fields (position, direction, post-step action) need to be either promoted or dropped
• The static std::ofstream and static std::mutex in the executor are a debugging approach and need to be replaced with proper output via OpticalStepParams::records()