Improve distinction between layout commands and operations

[rickwporter]

RATIONALE

For layout, improve clarity between command and operation nodes. This

CHANGES

1. Broke LayoutFields into two separate fields
   • Created CommandField for command (branch) nodes
   • Created OperationField for operation (leaf) nodes
2. Separated data_to_node() into separate branch/leaf nodes
   • parse_layout_command()
   • parse_layout_operation()

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.53%. Comparing base (57623ea) to head (be5023d).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   99.58%   99.53%   -0.06%     
==========================================
  Files          33       33              
  Lines        3821     3838      +17     
  Branches      697      697              
==========================================
+ Hits         3805     3820      +15     
- Misses          0        1       +1     
- Partials       16       17       +1     

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