✨ Natural spawning — all 7 structures

Added StructureHelper.levelFoundation(): before any other building happens, it now levels the footprint to a flat pad at the resolved ground Y — filling gaps below (so floors don't float over a dip) and clearing bumps above (so a mound or tree stump can't poke through a wall), capped at 6 blocks either way so a badly out-of-range forced spawn can't dig an enormous hole. Wired into all 7 structures' spawn().