Region shape parameters (CIRCLE)
================================

                 radius (chunks)
       <------------------------------>
        ____________________________
       /                            \
      /        ________________      \
     /        /                \      \
    |        /     centerRadius  \     |
    |       |    <------------>   |    |
    |       |          *          |    |    * = (centerX, centerZ)
    |        \                   /     |
     \        \________________/      /
      \                              /
       \____________________________/

  Players land in the annulus between centerRadius and radius.
  Both are measured in CHUNKS (1 chunk = 16 blocks).

  weight = 1.0   -> uniform distribution over the annulus.
  weight > 1.0   -> bias toward the centre.
  weight < 1.0   -> bias toward the edge.

SQUARE / RECTANGLE shapes use the same parameters with a square / rectangular
outer boundary. The centerRadius "donut hole" is always circular by default.

CIRCLE_NORMAL / SQUARE_NORMAL shapes apply a Gaussian falloff instead of a
flat distribution; `weight` controls the standard deviation.
