[extensions] load extensions on the IO thread

Editor assessment

The subject states the remedy, extensions are loaded on the IO thread, with a bracketed [extensions] tag standing in for a proper scope. What is missing is the defect this fixes, presumably a thread-assertion crash or unsafe access, and no body supplies the failure mode or the constraints that dictate the thread choice. The fix type is right, no footers appear, and hygiene suffers from the bracket-prefix habit. The change is accurately described for core contributors while the rationale remains locked in the diff, which keeps the message at acceptable.