2layer: 64x64 binary maze generation in 10 seconds, tech demo

Generated in ~10 seconds, this example output demonstrates scaling up to large grids. The trick is to use multi-scale reasoning, thinking about routes through large slabs independently of routes within slabs. This initial prototype used a two-level setup with an 8x8 grid of 8x8 grids. No interesting aesthetic constraints were added, and the length-bounding logic has some fatal off-by-one errors that cause it to miss some connections right now.