Encode time-stepping mode in SimulationMetaData and dispatch neighbor loops

[AhmedSalih3d]

Motivation

• Centralize the time-stepping strategy into the simulation metadata to make the simulation API cleaner and enable compile-time dispatch on stepping mode.
• Remove the ad-hoc SimTimeStepping argument from RunSimulation to simplify caller code and example scripts.
• Separate the neighbor-loop logic for the two stepping strategies so the main loop is simpler and easier to maintain.

Description

• Add a TimeSteppingMode abstract type and concrete types SymplecticTimeStepping and SingleNeighborTimeStepping, and extend SimulationMetaData with a TMode type parameter.
• Provide a default SimulationMetaData constructor that defaults TMode to SingleNeighborTimeStepping to preserve prior behavior.
• Remove the SimTimeStepping parameter and the TimeSteppingMode field from runtime metadata, and update RunSimulation signature to accept the SimulationMetaData with the time-stepping mode encoded in its type.
• Introduce InitialNeighborLoop! and HalfStepNeighborLoops! methods specialized for SymplecticTimeStepping and SingleNeighborTimeStepping, and refactor SimulationLoop to call these helpers instead of branching inline; update README and example scripts to reflect the new metadata-driven configuration.

Testing

• No automated tests were present or modified in the repository, so no automated test suite was run as part of this change.
• Local type-level compilation/inspection was used to ensure the new dispatch-based code paths resolve, with no compile-time errors observed.

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Codex Task