Pool Initialization

_beforeInitialize enforces two conditions before the pool can exist:

1. currency0 and currency1 are the wrapper and underlying currencies, in the order fixed by wrapZeroForOne (InvalidPoolToken).
2. fee == 0 (InvalidPoolFee).

wrapZeroForOne is an immutable set in the constructor to underlyingCurrency < wrapperCurrency (by address). It records which token is currency0:

• wrapZeroForOne == true: currency0 is the underlying, currency1 is the wrapper. A zeroForOne swap wraps.
• wrapZeroForOne == false: currency0 is the wrapper, currency1 is the underlying. A zeroForOne swap unwraps.

This maps a swap direction to wrap or unwrap deterministically, regardless of how the two token addresses sort.