The Downtown Roseburg Association will host a smish-smash of activities Saturday to show off the town’s center and drum up interest in building a plaza. The association coined the first-time event “Flash Plaza” to suggest the spontaneity of a flash mob. “All of a sudden, it just happens,” the association’s executive director, Roxana Grant, said today. Underlying the impulsive appearance, however, is a desire by the association to promote a plaza, she said. An assortment of free events, including chess matches and a paper airplane contest, are intended to showcase things people might see in a town plaza, Grant said. …