Previously the cycle validator rejected any link whose source had a
vector index >= target's, which silently killed legitimate connections
between nodes added in the wrong drop order.
Switch to a DFS over source_ids: an edge from->to creates a cycle iff
`from` already (transitively) depends on `to`. topo_sort runs after
each topology change so the vector ends up in a consistent order
regardless of how nodes were inserted.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
egutierrez
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