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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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.01%. Comparing base (b191cc1) to head (f8bd2b2).

Files with missing lines	Patch %	Lines
...riable-sensitivity/variable_sensitivity_domain.cpp	97.67%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8805   +/-   ##
========================================
  Coverage    80.01%   80.01%           
========================================
  Files         1700     1700           
  Lines       188345   188381   +36     
  Branches        73       73           
========================================
+ Hits        150696   150731   +35     
- Misses       37649    37650    +1     

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[Copilot]

Pull request overview

Improves three-way merge behavior under --vsd-structs every-field by making merges field-sensitive for structs and recursively descending into nested struct fields, while conservatively treating arrays as wholly modified.

Changes:

• Added recursive path collection to identify leaf-level modified fields within structs (and conservative handling for arrays).
• Updated three-way merge of function returns to assign only modified sub-paths rather than entire symbols.
• Added a regression test covering field-sensitive merges for struct fields.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/analyses/variable-sensitivity/variable_sensitivity_domain.cpp	Adds recursive modified-path discovery and uses it to selectively assign modified fields during three-way merge.
regression/goto-analyzer/variable-sensitivity-assign-aware-merge-struct-02/test.desc	Adds a regression driver for --three-way-merge with --vsd-structs every-field.
regression/goto-analyzer/variable-sensitivity-assign-aware-merge-struct-02/main.c	Adds a C test exercising field-level preservation across merges.

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