Fix world border edge placement

Right now it just refuses to place a block that's adjacent to the world border. This is because you can't place against a block that's outside the world border. Instead, the check canPlaceAgainst should just verify that it's inside the worldborder. This will work in both cost calculation and execution, instead of being unable to place adjacent at all, it will properly calculate whether or not it's possible to place, if there's another position to place against that's inside the border.

Locate the canPlaceAgainst check and trace how it is used during cost calculation and execution. Verify the world-border condition for positions adjacent to the border, then confirm that placement is rejected only when no valid position inside the border can be used.

Written by the indexing model from the issue text.