Adds the blocking-Io async/await example with a seeded empty .exit marker.
The example fails today (Io protocol + context.io + async/await not yet
implemented); the next commit lands the Io interface + blocking impl +
both __sx_default_context materializers + push-inherit fix to turn it green.
Worker is a lambda with annotated params (the verified B1.2 idiom);
named-fn workers are deferred pending a :: callable-param feature.
