Project merging: ensure nodes are sorted

There are currently cases in project merging where matches are non-deterministic, meaning nodes can be assigned different UUIDs at different times.

 # Source: trigger->a, a->b, b->c, b->d
 # Target: trigger->x, x->y, y->z, y->q
 # Special case: node 'b' has both parent and children changed
 # Should map: a->x, b->y, c->z, d->q

c and d can both map to z or d, and which is chosen depends on the order they happen to sit in the array.

The original algorithm proposed a sorted queue of nodes, which would ensure a deterministic order. Sorting can be done hierarchically, top-down and left-to-right. Child order should be determined by x/y position if present, or node id (not uuid) if it is not.

See thread: https://github.com/OpenFn/lightning/pull/3582#issuecomment-3370018765

Locate the project-merging implementation and review the discussion in PR #3582, especially the proposed sorted queue. Define the hierarchical, top-down and left-to-right ordering, using x/y position or node id as described, then verify that the example mapping is deterministic regardless of array order.