Add Multi-Particle Transport Infrastructure and Abstractions

Goal

Design and implement the shared infrastructure needed for coupled multi-particle transport in MC/DC.

Scope

This issue should define the interfaces and abstractions for transporting multiple particle types in one simulation, including cases where one particle type produces another.

Areas to consider:

• Particle-type registration and selection.
• Shared vs. particle-specific transport behavior.
• Secondary particle creation and banking.
• Source, bank, tally, and output handling across particle types.
• Interfaces used by physics models and transport algorithms.
• Compatibility with future electron, proton, photon, and heavier charged-particle support.

Acceptance Criteria

• Multi-particle infrastructure requirements are specified.
• Core interfaces and abstractions are proposed.
• Dependencies for particle-specific transport issues are identified.
• Initial tests and verification needs are outlined.
• Design is reviewed for extensibility before particle-specific implementations depend on it.

Notes

This is a framework-level sub-issue. The assignee should expand the detailed design, implementation plan, and acceptance criteria before implementation.