New sports coupes arrive like summer lightning and depart like uneaten guacamole: ignored, slowly fading to brown, awaiting the inevitable flush. The six-year-old Mazda RX-8 languishes through 2008 with sales the Hubble telescope wouldn’t register: just 2591 units through the end of August. Mazda isn’t giving up, however. The Hiroshima headquarters considers rotary engines integral to its