NEW YORK (AP) — Retailers are kicking off the unofficial start of the holiday shopping season on Friday with a bevy of discounts and other enticements. But executives are growing concerned with a spending slowdown that could temper sales on the day after Thanksgiving as well as throughout the holidays. Shoppers, powered by a solid job market and steady wage growth, had demonstrated a resilience that confounded economists and ran counter to sour sentiments expressed in opinion polls.