Function two_opt 

pub fn two_opt(
    solution: &mut Solution,
    instance: &SolomonInstance,
) -> Result<GpuSearchReport, GpuEvaluationError>

Runs GPU-selected 2-opt independently on every route in a solution.

Routes are validated individually; this does not check that the solution visits every customer or satisfies fleet and capacity constraints. Reversal preserves route membership and demand. The entire solution is unchanged if any route fails validation, CUDA execution, or move verification.