Challenge 15: Reject Unbalanced Deposit

A deposit that changes the reserve ratio effectively moves the pool price.

Task

Implement is_proportional_deposit.

Hint

Instead of dividing, compare cross products:

x_deposit * y_reserve == y_deposit * x_reserve

Use u128 for the multiplication.

Example

For reserves (1000, 2000) and deposit (250, 500), the ratio matches exactly, so return true.