Preserve Rust parser state across tokenizer runs and stop cleanly when
a parser-blocking script has to execute. Thread the pending script back
through the existing C++ parser entry point so document.write(), input
insertion points, and script bookkeeping continue to use the normal
LibWeb machinery.
Add the fragment parser setup needed by innerHTML and contextual
fragment parsing, including context elements, form ownership, tokenizer
state selection, text coalescing, and foreign-content integration.
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