Fix variable initialization order in snapshot-harness aliasing pointers

[tautschnig]

The root cause was in the topological sort implementation. The dfs() method enforced that seen and inserted sets must be empty on entry (via PRECONDITION) and would clear them on exit. However, topological_sort() calls dfs() multiple times in a loop for each item in the input collection. This meant only the first item was sorted correctly; subsequent items would fail the precondition or produce incorrect results.

The fix moves the set clearing logic to the beginning of topological_sort(), ensuring a fresh DFS traversal for the entire collection while allowing the DFS to maintain state across recursive calls within a single item's dependency graph.

Co-authored-by: Kiro autonomous agent

Fixes: #4978

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