Fix: Invariant emitted in swap event is incorrect

[Alexander]

Description

In swap, the next invariant is computed in the invariant check:

portfolio/contracts/Portfolio.sol

Lines 526 to 532 in 7320ddd

 (validInvariant, nextInvariantWad) = checkInvariant( 
     args.poolId, 
     iteration.prevInvariant, 
     iteration.virtualX, 
     iteration.virtualY, 
     block.timestamp 
 ); 

However, while this nextInvariant is the valid invariant without the fees being considered, the actual invariant should include the fees that were added to the pool, which is not updated until after this amount is computed:

portfolio/contracts/Portfolio.sol

Lines 552 to 556 in 7320ddd

 if (_state.sell) { 
     iteration.virtualX = nextIndependent; 
 } else { 
     iteration.virtualY = nextIndependent; 
 } 

Update the nextInvariantWad to use the nextIndependent value for either X or Y reserves.

[Alexander]

Closing for now. Emits the pre-fee invariant.