Function round 

pub fn round<N: DomNavigator>(
    _context: &mut DynamicContext<'_, N>,
    args: Vec<XPathValue<N>>,
) -> Result<XPathValue<N>, XPathError>

Implements fn:round - returns the nearest integer to the argument.

Rounds half values away from zero (e.g., 0.5 -> 1, -0.5 -> -1). The function preserves the numeric type of the input.