debug block

Internal worldgen placeholder, not a finished building block. This is the first replacement slot used by several decoration classes, so it only gains a real identity when a template asks the generator to swap something into slot one.

In shipped data that first slot is reused for very different jobs: tree canopies route it through their leaf block, simple decorations use it as the first entry in their replacement list, and rock decorations place it where ore pockets should appear. If you spawn it by hand, it stays a debug cube because no decoration context is present to resolve the swap.