Function validate_spatial_periodic_boundary_connection 

pub fn validate_spatial_periodic_boundary_connection<I>(
    ports: &[BoundaryPhysicalPortContract<I>],
) -> Result<CartesianPeriodicBoundaryIdentification, SpatialPeriodicBoundaryViolation>
where
    I: Eq,

Validate and derive one exact Cartesian translation identification.

Unlike an ordinary conserving connection, the two supports are expected to be noncoincident. The exact parent and opposite-side contract makes the translation unique, so no tolerance or duplicate vector enters canonical meaning.