it's useful to have an immutable stub around for the sake of feeding back dummy read values, but for write values it has to barf instead of being quiet.
There's still some issues with LightArray which I don't currently have a solution for, but I'm thinking about separating light storage from chunks anyway.
copy-on-write and zero-layer SubChunk objects are much easier to manage and have less cognitive overhead.
obviously, the goal is to get rid of EmptySubChunk completely, but right now it does still serve a purpose (filling in dummy values for reading out-of-bounds on chunks), and that's a problem that takes a little more work to fix.
