Fix three-way-merge to be field-sensitive with --vsd-structs every-field

[tautschnig]

For structs with struct-typed fields, merging now recursively descends to identify leaf-level modifications. Array handling uses a conservative approach (marking entire array as modified) since enumerating all indices is not practical.

Co-authored-by: Kiro autonomous agent

Fixes: #6272

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[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.01%. Comparing base (76dd20f) to head (a4f1655).

Files with missing lines	Patch %	Lines
...riable-sensitivity/variable_sensitivity_domain.cpp	95.34%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8805   +/-   ##
========================================
  Coverage    80.00%   80.01%           
========================================
  Files         1700     1700           
  Lines       188249   188285   +36     
  Branches        73       73           
========================================
+ Hits        150615   150649   +34     
- Misses       37634    37636    +2     

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