New Canaan has lost a popular and friendly downtown presence, especially among young locals. Tango, a handsome 7-year-old golden retriever belonging to Phyllis Weinstein of Mackenzie’s, died Thursday, within days of an unexpected diagnosis of an aggressive cancer. A makeshift memorial has been set up in the South Avenue shop for locals to say goodbye. “He was like the mascot of the store,” said Jim Berry, a welcome face behind the counter. “He was always here, mingling with the kids.” Those kids include hundreds of Saxe Middle School students, who for years have carried on a tradition of depositing their school bags outside Mackenzie’s after school, and getting some candy or other items inside.