BuildGhost: a pre-placed cube actor (not runtime-spawned, to avoid
the SpawnActor Class-pin problem for this piece) referenced via
GhostBlockRef. Shown and moved to SnappedLocation each tick while
bBuildMode is on; hidden otherwise. Collision left enabled on the
ghost (couldn't find the right property path to disable it through
this tool -- BodyInstance.CollisionEnabled isn't a flat property);
minor known edge case where the raycast could self-hit the ghost,
not addressed yet.
58794dbd25