Refactor|Optimize|World: Preprocess sector cluster containment for dynamic plane mapping

Sector cluster containment and unique neighbor determination is now
processed only once when the cluster is first dynamically mapped. If
the mapping is later invalidated (e.g., due to a plane height change)
this data is immediately available for reuse.

This should help to limit the performance impact of map hacks which
involve dynamic elements (e.g., HacX MAP05).