docs: document S003 arithmetic overflow behavior

[Joeloo1]

Summary

This PR documents the S003 arithmetic overflow/underflow detection behavior for production scale deployment.

Changes

• Added comprehensive rule documentation in docs/rules/s003-arithmetic-overflow.md
• Enhanced inline code documentation in arithmetic_overflow.rs and lib.rs
• Updated DOCUMENTATION_INDEX.md with S003 reference
• Added implementation summary document

Documentation Includes

• Detection rules (15+ patterns)
• Exclusions (test code, indexing)
• Remediation guidance
• Contribution notes
• Known limitations
• Output format specifications

Impact

• ~1,450 lines of documentation added
• Zero breaking changes (documentation only)
• Zero API changes
• Output formats remain stable

Closes #498

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