[Chess] from_fen: don't inherit phantom startpos history#1319
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GameState() defaults carry the startpos hash in hash_history[0] and the startpos board in board_history[0]. from_fen built on those defaults and then rolled them via _update_history, so every restored position claimed the startpos as its previous position - a phantom repetition entry and a wrong oldest observation plane. Clear both histories before updating. (cherry picked from commit 2d4b5a2)
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Summary
GameState()defaults carry the start position inhash_history[0]andboard_history[0].pgx.experimental.chess.from_fenbuilds on those defaults and then rolls them via_update_history, so every restored position claims the standard start position as its previous position:hash_history[1]contains the startpos Zobrist hash → a phantom repetition entry. Restoring the start position itself viafrom_fenimmediately reports one repetition; any game that later transposes back to the startpos-hash counts one repetition too many.board_history[1]contains the startpos board → the oldest observation history plane shows a position that was never played.Fix
Clear both histories before calling
_update_history, so the restored position starts with a clean history (matching the semantics of a position restored from FEN, which has no known past).