Added stadium_owner_id field to GameState to track who played the stadium:
- stadium_owner_id: str | None tracks the player who played the current stadium
- When a stadium is replaced, old stadium discards to OWNER's pile (not current player)
- Added stadium_owner_id to VisibleGameState for client visibility
- Updated existing test and added 2 new tests for stadium ownership
This fixes the bug where replacing an opponent's stadium would discard
to the current player's pile instead of the opponent's.
797 tests passing.
554178dc6e