Function get_beard_contribution 

fn get_beard_contribution(dx: i32, dy: i32, dz: i32, y_to_ground: i32) -> f64

Computes the beard density contribution for a point near a structure piece.

dx, dy, dz are the distances from the query point to the piece for kernel lookup. y_to_ground is the vertical distance from query point to the piece’s ground level.