Contested Space

How it works

1. Two different unfinished islands are checked together, not just one at a time.
2. If sending a particular cell to sea would leave one of them without enough reachable room to hit its own target — even though the OTHER island isn't the one being reasoned about — that cell must be land instead. This is the cross-island version of the plain reachability check, which only ever looks at one island's own limit.