fix: reassigning Node.edge no longer detaches the node from its parent

[steps-re]

Split out of #234 at @mmore500's request, so the parent/children bookkeeping change can be discussed on its own.

What it changes

Node._set_edge() (the setter behind node.edge = new_edge) removed the node from its parent's _child_nodes list as a side effect of swapping in a new Edge:

if self._parent_node is not None:
    try:
        self._parent_node._child_nodes.remove(self)
    except ValueError:
        pass

This patch removes those lines. A node's tail_node is derived from head_node._parent_node, not stored on the edge, so swapping the edge object has no bearing on where the node sits in the tree. As written, node.edge = new_edge silently dropped the node from its parent's children while node.parent_node still pointed at the old parent, leaving the tree in an inconsistent state.

The change is covered by a regression test (test_edge_setting_does_not_corrupt_parent_child_bookkeeping) asserting that after reassigning a child's edge, the parent still lists the child and the child still points back at the parent.

Why it's split from #234

#234 is a batch of small, independent static-analysis fixes. This one touches live tree-mutation semantics rather than a self-contained bug, so it warrants its own review thread. The rest of #234 (including the unrelated distance_from_root() None-parent fix in the same file) stays there.

Open question from @mmore500

Should _parent_node also be reset when the edge is reassigned? This PR only stops the node from being removed from the parent's _child_nodes list, it does not touch _parent_node. If the intended semantics of reassigning an edge are "detach this node from its current parent entirely," then both sides of the link (_parent_node and the parent's _child_nodes) should be cleared together, rather than the current half-and-half. Happy to adjust in whichever direction you prefer.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.82%. Comparing base (e708549) to head (9c829d1).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
+ Coverage   88.81%   88.82%   +0.01%     
==========================================
  Files         114      114              
  Lines       12963    12975      +12     
==========================================
+ Hits        11513    11525      +12     
  Misses       1450     1450              

Flag	Coverage Δ
unittests	88.82% <100.00%> (+0.01%)	⬆️

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[steps-re]

Rebased on main to kill the conflict. Was just a collision with a sibling PR that dropped TestNodeDistanceFromRoot in the same spot. Kept both tests, squashed to one clean commit.