Rule: Math lints for unchecked u128 + Q64.64 + slippage/invariant bounds

[Cass402]

Summary

Warn on unsafe arithmetic in high-value financial code paths, especially unchecked u128 operations and missing bounds in AMM logic.
Related to #3

Why it matters

•	Unchecked u128 mul/div can overflow silently.
•	Fixed-point Q64.64 math can lose precision or overflow without guard rails.
•	Missing slippage or invariant checks can allow value extraction or pool imbalance.

Proposed Approach

•	Warn on * or / with u128 unless checked_*, mul_div, or equivalent safe helpers are used.
•	For Q64.64 ops, suggest explicit rounding and overflow handling.
•	In swap/AMM code paths, detect absence of a slippage bound (require!(amount_out >= min_out)) or invariant check before state mutation.

Examples

Flag:

let k = a * b; // a, b are u128

Pass:

let k = a.checked_mul(b).ok_or(Error::Overflow)?;