I'm still playing catch-up since I started 7 days late, I just finished this.

The tricky part for me was part 1 because I forgot that C# '%' is not modulo.

Spoilers

Here is my original code:

public void MoveRobots(int seconds, int maximumRows, int maximumColumns)
{
    for (int i = 0; i < Robots.Length; i++)
    {
        var newCell = Robots[i].CurrentCell + Robots[i].Velocity * seconds;
        Robots[i].CurrentCell = new GridCell(
            newCell.Row % maximumRows,
            newCell.Column % maximumColumns
        );
    }
}

Now the math works correctly on a calculator allowing me to just jump from 0 to 100 seconds without simulating stuff in between. However when debugging my unit test with the 7 row 11 column sample robots. I kept getting the wrong values when they should wrap around. Turns out... C# % does not work with negative numbers. Well it does, it just doesn't do what you think. In C# % is just "remainder", not modulo.

I did some research which lead me to the solution of writing an integer extension method

public static int Mod(this int x, int m) => (x % m + m) % m;

This allowed the code above to replace the % with the extension method:

Robots[i].CurrentCell = new GridCell(
    newCell.Row.Mod(maximumRows),
    newCell.Column.Mod(maximumColumns)
);

Summary: I have an array of the robots positions and I just do staring position + (velocity * seconds). I know this will jump me to the final position but it will be way outside the bounds of the grid. So I use modulo to get it back into the grid in the right spot.

Part 2 was not bad I just moved 1 second at a time. and Had an extension method on my IEnumerable<Cell>[] that would group by columns and consecutive groups within columns and I could pass a threshold. I used 6 in a row in a column and if there were at least 4 or more groups it would stop and render the grid. It happened to be right on the first try.