Handle Surrogate-Target Immaturity for the Freshest Impressions

The 7-day session-return surrogate model can only score impressions once 7 days have passed. The ranker retrains nightly on the last 30 days of impressions.

1. What happens if the freshest impressions (the last 7 days) are simply excluded from every nightly retrain?
2. What happens if, instead, the freshest impressions are included with a placeholder/default surrogate value?
3. Propose a concrete policy that avoids both failure modes, and explain the trade-off it makes.