Function union_polygon 

pub fn union_polygon(poly1: &Polygon, poly2: &Polygon) -> Result<Vec<Polygon>>

Computes union of two polygons

Returns a simplified union polygon. This implementation handles:

• Disjoint polygons (returns both)
• One polygon containing the other
• Overlapping polygons (simplified boundary merge)

Arguments

• poly1 - First polygon
• poly2 - Second polygon

Returns

Vector of polygons representing the union

Errors

Returns error if polygons are invalid