edge
Weight of the edge from from to to. Equals voxelCostOf(to) * stepLength, where stepLength is:
1.0 for orthogonal moves (one axis differs)
√2 for face-diagonal moves (two axes differ)
√3 for body-diagonal moves (all three axes differ)
The caller is responsible for ensuring to is a neighbor of from under the movement rule; this function does not validate that.