Hexagon can't coexist with Start or End

[hcs64]

It isn't possible (at least in the UI) for a grid intersection to be a Start and also have a Hexagon on it, same with End.

For instance, this puzzle can't currently be implemented literally.

I can't think of a puzzle that would require this to force a solution, since you can always put the hexagon on one of the paths leading to the intersection instead, but having it on the intersection itself avoids giving a hint as to which direction to go. It is only mechanically meaningful to have the hex if there are multiple exits/entrances and you might need to pass by them, but aesthetically (as in the above-linked puzzle) it is also desirable.

This might be tangentially related to #16.