check_return_path demands a ground via within 2 mm of every high-speed layer change, and no router or tool places one. Freerouting adds none, the KRT GND-via option warned no GND net was found and is not wired to constraints, and stitching only does grids, so every inner-layer DDR/USB/PCIe transition on linux-computer, network-switch and zcoupon is born a return-path finding and fixed by hand.