Watchers have inconsistent state inside `watched` callback

[prophile]

Consider the following code:

let watcher
const a = new Signal.State(0, {
  [Signal.subtle.watched]: () => {
    console.log(Signal.subtle.introspectSinks(a))
    console.log(Signal.subtle.introspectSources(watcher))
  },
})

watcher = new Signal.subtle.Watcher(() => {
  console.log("notified")
});
watcher.watch(a);

Inside the watched callback, a is reported as having no sinks, but watcher does have a as a source – a visibly inconsistent state.