## Summary
- Adds an arithmetic invariant check (`slot + size == 151`) to
`check-storage-layout.sh` that catches cases where a new state variable
is added without shrinking `__GAP` accordingly
- The existing snapshot-diff check alone could pass with a wrong gap
size if the snapshot is updated to match — this new check prevents that
- Updates the negative test to also accept the new `__GAP invariant
violated!` error message
## Test plan
- [x] `check-storage-layout.sh` passes on the current correct layout
- [x] `check-storage-layout-negative.sh` passes — the bad layout
contract (slot 107 + size 45 = 152) correctly triggers the invariant
failure