Function predict_grid_position 

pub fn predict_grid_position<F: Float>(
    grid: &HashMap<GridIndex, Point2<F>>,
    idx: GridIndex,
) -> Option<Point2<F>>

Predict a grid corner’s position from its cardinal neighbors.

Uses midpoint averaging:

• Horizontal: 0.5 * (P(i-1,j) + P(i+1,j))
• Vertical: 0.5 * (P(i,j-1) + P(i,j+1))

Returns the average of available predictions, or None if no complete neighbor pair exists (need at least one horizontal or vertical pair).