Replace the spin_until in SVGScriptElement::process_the_script_element
with an async fetch that mirrors HTMLScriptElement's mark_as_ready
pattern. External SVG scripts now fetch and execute asynchronously,
matching Chromium's behavior.
For HTML-embedded SVG scripts, the parser pauses via the existing
schedule_resume_check infrastructure, extended to support SVG scripts
through a new pending_parsing_blocking_svg_script slot on Document.
For top-level XML/SVG documents, scripts execute when their fetch
completes; the load event is delayed via DocumentLoadEventDelayer which
the existing XMLDocumentBuilder::document_end already waits on.
53fa1b19f1