A green check in the section written to prove the rule is enforced

An island test asserted that cargo refuses an undeclared sibling crate reference. It passed on its first run — with error E0753, a syntax error, because the test inserted the `use` line above the module doc comment. The crate never parsed, so the import was never resolved and the property under test was never exercised. The accompanying assertion 'it refuses for the right reason' also passed, because it only required the output to mention the crate name, and the test's own inserted line was quoted in the error.