Fix SliceLayouts.CreateColumnLayout creating a row layout

[dalyIsaac]

The tests previously succeeded because:

• the SliceLayoutEngine record didn't expose the root area, which meant they were not included in the generated record Equals
• apparently casting ILayoutEngine to SliceLayoutEngine matters 🤷

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.55%. Comparing base (6b195f7) to head (2b948be).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1039      +/-   ##
==========================================
+ Coverage   80.47%   80.55%   +0.07%     
==========================================
  Files         280      280              
  Lines       11986    11988       +2     
  Branches     1373     1373              
==========================================
+ Hits         9646     9657      +11     
+ Misses       2166     2157       -9     
  Partials      174      174              

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