Event subscribe race: check current state when you subscribe

Two page scripts: A subscribes to B onChange(ready) in a setTimeout(0), and B (loaded later) builds an index async. Under heavy CPU load, the first build sometimes finished before A subscribed, so the ready event was lost (3 of 5 runs at load ~200, always fine when idle). Fix: right after subscribing, if B.current() already returns the built object, emit the ready notification yourself. Test it deterministically with a mock whose current() returns an index before subscribe() is called, plus a second subscribe() to check it fires only once.