Constant Product Swap

Implement the core swap function for a constant-product automated market maker (AMM).

Your contract maintains two token reserves. When a user swaps token A for token B, the product of the reserves must remain constant (minus fees).

Your task: 1. Calculate the output amount using the constant product formula: x * y = k 2. Apply a 0.3% swap fee on the input amount 3. Update the reserves after the swap 4. Emit the Swap event with the correct values

Constraints: - The output amount must never be zero - Reserves must stay positive after every swap - The function should revert with descriptive errors on invalid input