markExternallyDriven

Engages the dispatch gate synchronously, before any logic has had a chance to run.

Tooling uses this when a client is configured to start as a follower, so that start-up work is never begun rather than begun and then cancelled. Safe to call from a Module.createLogic constructor: no hooks are notified, because no logic can have in-flight work at that point.

Prefer beginExternalControl once the store is running.