Function check_fixture 

pub fn check_fixture<T>(cx: &mut Cx, fixture: T) -> Result<()>
where
    T: CitizenRuntime,

Checks one runtime fixture’s read-construct round trip and failure paths.

Encodes fixture to its constructor encoding, derives a deliberately wrong version argument, and delegates to check_value_fixture_with_wrong_version so the gate also rejects bad versions alongside the round-trip, capability, and arity assertions.