refactor: route i128 hot paths through safe_math wrappers

[ayo-ola0710]

What was done

A new safe_math module was introduced to centralize overflow-resistant arithmetic via a SafeMath trait that exposes s_add, s_sub, s_mul, and s_div operations returning a Result<Self, RevoraError>. Using this new module, hot paths in the codebase—specifically the compute_share calculation, supply cap and revenue accrual updates in deposit_revenue and report_revenue, and the reconcile_audit_summary aggregation loop—were refactored to replace silent saturating_* and raw checked_* patterns with explicit error-handling boundaries. Finally, comprehensive unit tests were added to safe_math.rs, and the strict #![deny(clippy::arithmetic_side_effects)] lint was enforced across all touched files (lib.rs, security_assertions.rs, and safe_math.rs) to guarantee future arithmetic safety.

Close #484

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