decreases clause does not support impure specs

```fstar
fn rec foo (x: ref nat)
preserves live x
decreases !x
{
if (!x > 0) {
x := (!x) - 1;
foo x;
}
}
```

Use the F* reproducer in the issue as the starting case, focusing on how decreases clauses handle the impure expression `!x`. Reproduce the failure, locate the decreases-clause checking path, and confirm completion when this example is accepted or produces the intended verification result.

Written by the indexing model from the issue text.