As a wobbly cease-fire keeps eastern Ukraine’s warring factions apart, Russia’s ruble is conquering new territory across the breakaway republics. In Donetsk, the conflict zone’s biggest city, supermarkets have opened ruble-only checkout counters to serve the fighters in camouflage lining up along pensioners. Bus and tram tickets come with a conversion from Ukraine’s hryvnia to the Russian currency. Gas-station workers …