fix: unify ForLoop traversal order between ASTWalker and ASTModifier

[Lil-Duckling-22]

Standardize ForLoop traversal to pre→condition→body→post order in ASTModifier to match ASTWalker. This ensures consistent behavior when inheriting from both classes and prevents potential bugs in optimizers that depend on traversal order.

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[clonker]

Thank you for taking an interest! The original implementation produced a more natural numbering when it comes to variable names (since its for {pre}{cond}{post}{body}). Also everything that depends on the specifics of the for loop has to override the method anyways. Closing the PR based on this.