[Conventions] Refactor factor_rewriter to use structured bindings for expr/sign pairs

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This PR refactors factor_rewriter.cpp to use C++17 structured bindings instead of accessing .first and .second members of std::pair(expr*, bool).

Changes

File: src/ast/rewriter/factor_rewriter.cpp
Type: std::pair(expr*, bool) representing expression and sign indicator
Naming: [e, sign] - descriptive names for expression and sign

Modified Functions

1. factor_rewriter::mk_adds() - 3 refactoring sites

• Line 163: Main loop processing additions

  • Before: bool sign = m_adds[i].second; expr* _e = m_adds[i].first;
  • After: auto& [e, sign] = m_adds[i];
  • Eliminates 2 intermediate variables, uses e directly
  • Updates all references within loop body (lines 167-188)

• Lines 198-202: TRACE output loop

  • Before: m_adds[i].second and m_adds[i].first
  • After: auto& [e, sign] = m_adds[i];
  • Cleaner trace output code

2. factor_rewriter::mk_muls() - 1 refactoring site

• Line 210: Loop building multiplication terms
  • Before: m_adds[i].first
  • After: auto& [e, sign] = m_adds[i];
  • More readable access pattern

Benefits

1. Readability: e and sign are semantically meaningful vs generic .first/.second
2. Code Simplification: Eliminates intermediate variable assignments
3. Error Reduction: Reduces chance of confusing .first and .second
4. Modernization: Adopts C++17 idiom (Z3 uses C++20)
5. Consistency: Similar refactoring pattern used in 12 previous PRs

Code Statistics

• Lines changed: 17 (+10 added, -10 deleted, net +0)
• Intermediate variables eliminated: 2 (sign, _e)
• Sites refactored: 4 total (3 in mk_adds, 1 in mk_muls)
• Functions modified: 2

Testing

• No functional changes to arithmetic rewriter logic
• All existing behavior preserved
• Original semantics maintained for polynomial factorization

Context

This is part of an ongoing effort to modernize the Z3 codebase with C++17/C++20 features. Previous structured bindings refactorings:

• PR Specify python version required in readme #1-9: SAT/SMT subsystems (103 sites)
• PR fp.isSubnormal is buggy #10: sat_th.cpp enode_pair patterns (4 sites)
• PR Corrected typo: interger -> integer #11: smt_context.cpp clause/assumption pairs (4 sites)
• PR Cleanup: avoid defining reserved identifiers #12: theory_bv.cpp variable pairs (3 sites)

Total Progress: 114+ sites refactored across 16 files

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