fix(StdStorage): distinguish derived return values from missing slots

[developer3516]

Motivation

Closes #140.

deal() fails on tokens whose balance is computed rather than stored. The original report uses an Aave aToken, whose balanceOf returns the stored scaled balance multiplied by a liquidity index:

balanceOf(user) == scaledBalance(user) * getReserveNormalizedIncome() / RAY

stdStorage.find() locates a slot by requiring that the value held in the slot equals the value returned by the call. For a scaled-balance token that is never true, so the search fails with:

stdStorage find(StdStorage): Slot(s) not found.

That message is misleading. The balance slot is found — checkSlotMutatesCall correctly reports that mutating it changes balanceOf. It is rejected one step later by the value-equality check, and the user is told nothing about why.

Solution

Report the two failures separately. When the search fails, probe whether any recorded read slot actually influences the return value:

• No slot influences it → unchanged, "Slot(s) not found." This is the reflection-token case from bug(StdCheats): deal() hangs on reflection tokens #740, whose regression test is untouched.
• A slot influences it but none hold it → a new message naming the two realistic causes:

stdStorage find(StdStorage): Slot(s) affect the return value but none hold it.
Target may derive the value (e.g. rebasing token) or pack it (try enable_packed_slots()).

Implementation notes:

• The probe lives in a helper rather than a flag threaded through the search loop. find() is already at the EVM stack limit — adding a single local there fails to compile with Stack too deep.
• The probe only runs on the failing path, which reverts anyway, so successful lookups are unaffected.
• Short bytes/string returns are excluded. There a word-wise mismatch says nothing about how the value is produced, so test_RevertStorageFindRestoresFailedShortBytesProbe keeps the original message.

Testing

test_RevertFindOnScaledBalanceToken adds a minimal aToken-style mock (stored balance × index) and asserts the new message. It mirrors the structure of the existing test_RevertFindOnReflectionToken.

forge test: 207 passing, 0 failing. forge fmt --check clean.

This does not make deal() work on such tokens — that would require inverting the token's own accounting, which is out of scope. It tells the user what happened so they can reach for vm.store or the underlying scaled-balance accessor.