analyze_what_if
ChatGPTUse when the user asks "what if I had played X?" at a specific move in a game already loaded (PGN or prior storyboard analysis_id). Validates the exact alternative SAN, builds a Stockfish continuation, compares to the played move, and estimates Maia naturalness at target_elo. Never substitutes a different move — illegal alternatives return an explicit error. Requires alternative_move_san, target_elo, and (pgn or analysis_id) plus (anchor_ply or move_label).






