refactor(benchmarks): convert unsized integer types to explicit-width types

[mvillmow]

Summary

• Replace bare int loop counters and sizing variables with int32_t (or size_t for reserve()/capacity calls) in all 9 benchmark .cpp files
• Add #include <cstdint> to each affected file
• Follow-up from refactor: Convert unsized integers to sized types (int32_t, uint32_t, size_t) #65; benchmark files were inadvertently excluded in that implementation

Test plan

• just format-check passes (verified — no formatting diffs)
• No functional logic changed — explicit-width types are semantically equivalent for the value ranges used in benchmarks

Closes #175

🤖 Generated with Claude Code

[github-actions]

✅ Dependency Audit

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High	0
Medium	0
Low	0

See the Security tab for detailed findings.

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[github-actions]

Security Scan Results

• ❌ Secret Scanning: Potential secrets found
• ✅ SAST: Completed (check Security tab for details)
• ✅ Dependency Scanning: Completed
• ✅ C++ Static Analysis: Completed
• ✅ Docker Image Scanning: 0 high, 22 medium vulnerabilities (acceptable)

Recommendations

• Review findings in the GitHub Security tab
• Check artifact uploads for detailed reports
• Address critical Docker vulnerabilities immediately

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Workflow: Security Scanning