Add iframe support: srcdoc/src content fetching, independent pipeline
rendering, style isolation, dimension attributes, and UA stylesheet
defaults. Includes code review fixes for recursion guard, dimension
capping, alpha compositing, spec-compliant u32 parsing, base URL
correctness, font loading, and golden test infrastructure that exercises
the full iframe rendering pipeline.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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