…ification

- Add add, sub, mul, div, pow, sqrt with checked overflow protection
- Revert on overflow or underflow across all operations
- Add SMT proofs via Z3 verifying no silent overflow for any input
- Add signed integer support for collateral and debt calculations
- Add fixed-point decimal math at 18 decimals with overflow safety
- Replace all raw arithmetic across 16 contract crates with library
- Add property-based tests validating against BigInt reference
- Keep checked math overhead under 10% vs unchecked baseline
- Add documentation with mathematical formulas and proof references

Closes Smartdevs17#518