Validate Forward Cold Starts

Validate frozen first-year priors against the completed forward RAPM refit for the same regular season:

uv run nba-validate-forward-cold-start --season 2025-26 --render-docs-page

The command reads the completed forward exposure-gated RAPM artifact, joins its frozen exposure_gated_cold_start prior rows to post-fit coefficients, and reconstructs actual player exposure from canonical RAPM stints. It writes:

artifacts/models/forward_cold_start_validation/<season>/<run_id>/

The artifact contains a player-level table, cohort metrics, an SVG scatterplot, and an integrity manifest. The docs page separates drafted, undrafted, realized-low-exposure, and rotation-exposure cohorts.