[yAudit-03] Fix swap fee accrual on realized gain

[clement-ux]

Summary

• accrue buy-side performance fees from the value of the actual settled input rather than the quoted notional spread
• pass amountIn through swap liquidity validation so exact-input and exact-output swaps use the same realized-gain basis
• update fork, fuzz, and invariant fee expectations and add a regression test for 18-to-6 decimal truncation below one base unit
• harden the Ethena and Lido invariant handlers so corpus mutations remain ABI-decodable in strict mode

Invariant harness

The invariant handlers accept full-word uint256 inputs and derive booleans or bound narrower values internally. This prevents mutated calldata from reverting in the Solidity ABI dispatcher before handler execution.

Testing

• Full suite excluding invariants: 531 passed, 0 failed, 3 skipped
• Ethena invariants with FOUNDRY_INVARIANT_FAIL_ON_REVERT=true: 256 runs, 12,800 calls, 0 reverts
• Lido invariants with FOUNDRY_INVARIANT_FAIL_ON_REVERT=true: 256 runs, 12,800 calls, 0 reverts

Audit report

Related yAudit issue: https://github.com/yAudit/origin-paxos-adapter-review/issues/3

[naddison36]

This is a good find.
I've checked the new method is economically equivalent without rounding issues. The new method is better as it handles the rounding.