Spinning a nested event loop to wait for a parser-blocking script blocks
the calling thread, can deadlock, and creates reentrancy hazards. Switch
to an event-driven pause/resume model, mirroring the prior
HTMLParserEndState refactor (df96b69e7a).
Three WPT document.write tests flip from Fail to Pass and are
rebaselined: all write an external script via document.write() followed
by inline content. With spin_until, control did not return to the caller
of document.write() between writing the script and observing its effects
so the test's order assertions saw a different sequence than the spec
mandates.
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