Fix (and test) parent-child relationships

[gselzer]

The main fix here is that passing the parent kwarg to a Node constructor wasn't actually setting the parent. The rest of the tests are just nice to have.

[Copilot]

Pull request overview

Fixes parent/child bookkeeping in the scene graph by ensuring parent= passed at node construction time results in a consistent parent→children relationship, and adds targeted tests around the Node relationship API.

Changes:

• Add tests/model/_nodes/test_node.py covering add/remove child behavior, constructor children= and parent= kwargs, reparenting, and __contains__.
• Update Node.__init__ to synchronize parent→children when parent is provided via kwargs.
• Prevent duplicate additions in Node.add_child, and simplify removal typing in _update_parent_children.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/model/_nodes/test_node.py	New tests validating parent/child invariants, signal emission, and reparenting behavior.
src/scenex/model/_nodes/node.py	Fix constructor-time parent= handling; add dedupe logic in add_child; adjust _update_parent_children removal.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (d8da025) to head (96aacdc).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   88.13%   88.38%   +0.24%     
==========================================
  Files          62       62              
  Lines        2950     3064     +114     
==========================================
+ Hits         2600     2708     +108     
- Misses        350      356       +6     

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