cross-era stat translation (S144 era-translation)
Inverts the Historical-Knox era norms: a season's primary per-game production has an era-relative z-score (vs starters at the same position that year); we map that z onto a TARGET era's peer distribution to answer "what would this season look like in 2023?". The norms artifact (data/nfl/era-norms.json) is the exact reference distribution KV-H uses, so translation and KV-H agree.
Coverage: era norms exist for 1999+ (the nflverse gamesPlayed era). Pre-1999 seasons have no norm and are honestly omitted from the translation — never fabricated.