Function watch 

pub fn watch(
    stores: &ServerStores,
    bus: &EventBus,
    config: &WatchConfig,
    report: impl FnMut(Watched),
) -> Result<usize>

Watch the event bus and build what lands.

Blocking, and deliberately so: the runner is one build at a time in the process that watched the event. A queue and a pool are the phase-5 problem; this is the part that has to be right, and “right” is easier to see when there is one of it.

Every outcome — built, skipped, failed — is handed to report rather than logged and dropped, so a caller can assert on what happened. A build that fails does not stop the watch: one repository’s broken recipe must not take every other repository’s builds with it.