Arc reversal

In some cases, we may want to reverse the arc between two uncertainty nodes. This should be userw-friendly.
It includes:
- Updating the frontend
- [Check for cycles]
- Flipping the arc graphically between the nodes
- Adding arcs from the parents
- Updating the probability table using the Bayes' theorem

The issue names no files, tests, or entry points. Start by locating the frontend arc interaction, cycle validation, parent-arc handling, and probability-table logic; review how uncertainty nodes and arcs are represented before deciding the design. Done means reversal is user-friendly, rejects cyclic results, flips the graphical arc, updates parent arcs, and recalculates the probability table using Bayes' theorem.

Written by the indexing model from the issue text.