Gamerule for the Nether Roof (Bedrock & Java)

I propose adding a gamerule that controls the build height on the Nether ceiling. This would:

• Preserve old Java worlds that already have builds on the ceiling, since the Java value could stay unchanged when updating the world to avoid problems.

• Fix Java–Bedrock parity, allowing players in Bedrock to build on the ceiling if they choose, making it a two version feature.

• Respect Mojang’s design choice, since the gamerule default value will still prevent building in the roof in new worlds in Bedrock and in Java, but it can be changed.