By Leigh Thomas PARIS (Reuters) – Belgian motorists are crossing the border into France to fill up on subsidised fuel that costs the Treasury billions of euros and is designed to shield French households from the full inflationary pain felt in some neighbouring countries. Subsidies and aggressive price caps on energy have helped shore up consumer spending in France, and with it economic growth. But maintaining these costly inflation-relief measures is getting harder, and weaning households off the state’s largesse may unleash a new wave of price rises down the line, some economists say. “We’ve…