[SMTChecker] Verify AMM invariant

https://github.com/xwvvvvwx/symtest-examples/blob/main/src/AMM.sol

The invariant of course depends on the token implementation.
There could be two ways to verify this:
- Add requires around the token calls to represent the post conditions of the token functions
- Trust a token implementation and use that

Start by reading the linked src/AMM.sol example and identify the AMM invariant and the token calls it relies on. Compare the two stated approaches—postcondition requires around token calls or trusting a token implementation—and define verification as complete only when the invariant can be checked under an explicit token assumption.

Written by the indexing model from the issue text.