Lululemon Athletica (NASDAQ: LULU) has shed nearly a fifth of its market value in a single month, following a September earnings update that lowered management’s full-year outlook for the second time this year. The decline does not belong entirely to Lululemon, as the broader retail sector also weakened sharply over the same period, complicating any straightforward read on the stock. The SPDR S&P Retail ETF (NYSEARCA: XRT) dropped 6% over the pa…