Add Proton Transport Capability

[ilhamv]

Goal

Add proton transport capability to MC/DC as part of the broader multi-particle transport effort.

Scope

This issue should define and implement the initial proton transport model, including required physics, data structures, coupling behavior, tests, and documentation.

Acceptance Criteria

• Proton transport requirements are specified.
• Initial implementation plan is defined.
• Required infrastructure dependencies are identified.
• Tests and verification needs are outlined.

Notes

This is intentionally a bare-bones sub-issue. The assignee should expand the technical scope and acceptance criteria before implementation.