Chosuke Yara, the head of the Ryukyu Independence Party, last month. “Independence is an idea whose time has come,” he said.Credit...Ko Sasaki for The New York Times NAHA, Japan — In a windowless room in a corner of a bustling market where stalls displayed severed pigs’ heads and bolts of kimono silk, Okinawans gathered to learn about a political idea that until recently few had dared to take seri