By Pavel Polityuk and Max Hunder KYIV (Reuters) – Russia tightened its grip on a key target in a battle for control of Ukraine’s eastern Donbas region while President Volodymir Zelenskiy pleaded for more Western arms to help Ukraine reach a battlefield “inflection point” and prevail in the war. Zelenskiy told Luxembourg’s parliament via videolink on Thursday that Russian forces now occupied about a fifth Ukrainian territory, with battle lines now stretching more than 1,000 km (620 miles). As the invasion heads for its 100th day on Friday, Russia says Washington is adding “fuel to the fire” wit…