[Bug][r256x64] Hearthlings not building slabs

I was noticing while recording tomorrow’s episode that it doesn’t seem like the validity of a slab building design is ever recalculated to weather it is a valid design for Hearthlings to build. If the first slab block of a design has no path to it, even if a valid path is built during the design via ladders (post design) the build will never be built. If valid ladders or path exists before the building will be built.

I also noticed that if you combine a valid building with an invalid building through design, into one building the valid portion will sometimes partially build and the invalid portion will never build.