op-challenger: Add rule to prevent defending the root claim.

op-challenger/game/fault/solver/rules.go

@@ -15,6 +15,7 @@ var rules = []actionRule{
 	onlyMoveBeforeMaxDepth,
 	onlyCounterClaimsAtDisagreeingLevels,
 	doNotDuplicateExistingMoves,
+	doNotDefendRootClaim,
 }
 
 func checkRules(game types.Game, action types.Action) error {
@@ -75,6 +76,13 @@ func doNotDuplicateExistingMoves(game types.Game, action types.Action) error {
 	return nil
 }
 
+func doNotDefendRootClaim(game types.Game, action types.Action) error {
+	if game.Claims()[action.ParentIdx].IsRootPosition() && !action.IsAttack {
+		return fmt.Errorf("defending the root claim at idx %v", action.ParentIdx)
+	}
+	return nil
+}
+
 func resultingPosition(game types.Game, action types.Action) types.Position {
 	parentPos := game.Claims()[action.ParentIdx].Position
 	if action.Type == types.ActionTypeStep {