Small improvement to day 24

This is the best I have been able to do so far without making vec2's x/y no longer readonly (and then having to clean up after myself each time I leave this scope, which can happen in 3 places...) and since it's a relatively simple change (doesn't kill the ergonomics too terribly), I'm gonna roll with it for now.

Part 2 110ms -> 75ms. I thought GetOrthogonalNeighbors was as fast as I could make it, but maybe there are better ways to get this same result...

src/24.cs

@@ -218,8 +218,9 @@ internal class Day24 : Day
             }
 
             // queue up all neighbor possibilities
-            foreach (var n in q.pos.GetOrthogonalNeighbors())
+            foreach (var dir in ivec2.FOURWAY)
             {
+                var n = q.pos + dir;
                 if (n == dest)
                 {
                     if (q.steps + 1 < minSteps)