The AbortController/AbortSignal globals were minimal stubs: AbortSignal had no
prototype and no throwIfAborted, and AbortController.abort() only set a flag
without firing listeners. Pages that read AbortSignal.prototype or call
throwIfAborted threw non-fatal TypeErrors.
AbortSignal is now a class with a prototype, aborted/reason, throwIfAborted(),
working addEventListener/onabort firing, and static abort()/timeout()/any().
AbortController.abort(reason) fires the abort event. Bootstrap-only, no perf
impact.
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