PWGLF: Replace anonymous enums with scoped enum classes in dndetaHi

- Convert all C-style anonymous enums to typed enum classes:
  EventCategory, TriggerClass, Species, TrackSign, AnalysisStep,
  ParticleType, PtVariation
- Add constexpr toInt<E>() helper to convert scoped enum to int
  for use with registry.fill() and vector indexing
- Rename ambiguous enumerators for clarity:
  kProtonMy -> Proton, kOPar -> Other, kNoPtVar -> None, kPtDw -> Down
- Update all usages throughout processData, processMC, and
  processMCNoV0Counting process functions
- Remove dead V0 loop code in processMCV0Counting (parameter
  fullV0s unused, inner loop had permanent `0 &&` guard)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyungjun-lee