NHL goalie goals-saved-above-expected (our own model)

Goals saved above expected (GSAx = expected goals against minus actual goals against, over shots faced), the natural derivative of our own shot-level xG model (see GET /v1/nhl/xg-leaders and packages/orchestrator/scripts/compute-nhl-xg.ts). Empty-net goals against are excluded from a goalie's own shots-faced count. Higher GSAx is better (more goals prevented than an average defended shot of that quality would predict).