By Simon Lewis KRAMATORSK/SLOVIANSK, Ukraine (Reuters) – In the eastern Ukrainian city of Kramatorsk, mechanic-turned-soldier Artchk helps shore up defences against imminent Russian attack while, nearby, farmer Vasyl Avramenko laments the loss of crops supplanted by mines. Shells are raining down steadily on Kramatorsk and its twin, Sloviansk, and they are poised to become the next frontline in Moscow’s offensive in the heavily industrialised Donbas. Their defenders are outgunned, but Ukrainians have repelled Russian-backed forces here before, the cities having been seized by pro-Kremlin separ…