Add safe-math wrapper module enforcing checked arithmetic across all i128 operations

[thlpkee20-wq]

Description

The codebase mixes raw arithmetic and checked_* patterns. Introduce a safe_math module exposing s_add, s_sub, s_mul, s_div returning Result<i128, RevoraError> and refactor hot paths (compute_share, accrual updates, reconcile_audit_summary) to use it.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: Revora-Contracts/src/lib.rs, Revora-Contracts/src/security_assertions.rs
• Clippy lint arithmetic_side_effects must be denied in the refactor scope

Suggested execution

• Fork the repo and create a branch
• git checkout -b refactor/safe-math-module
• Implement changes
  • Add safe_math.rs with helpers and unit tests
  • Replace raw ops in compute_share with safe_math
  • Enable #![deny(clippy::arithmetic_side_effects)] on touched modules
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test --all
• Cover edge cases
  • i128::MAX overflow attempts must surface ArithmeticOverflow via Result
• Include test output and security notes

Example commit message

refactor: route i128 hot paths through safe_math wrappers

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours

[ayo-ola0710]

@ayo-ola0710 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi
I would love to handle these pls

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ayo-ola0710 to this issue.

[grantfox-oss]

🦊 GrantFox — @ayo-ola0710 has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #484)
2. Your PR will be reviewed by the RevoraOrg maintainers

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[drips-wave]

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