Train The Combined Box-Score Prior RAPM

After both component forecasts have been built for a target season, run:

uv run nba-train-combined-box-score-prior-rapm --season 2025-26

The combined prior is a hard switch: the returning-player box-score forecast is used when a prior NBA box profile exists, and the preseason cold-start profile is used otherwise. The resulting prior table is frozen before the existing chronological prior-centered RAPM fit, regular-season holdout, and playoff evaluation.

This is an ablation, not an automatic Leaderboard promotion. It must improve the locked lineup-level metrics over the forward-lagged RAPM prior.