Finish the Rust implementation of the spec tree-construction algorithms
needed by the LibWeb test suite. Add the remaining table modes, foster
parenting, scope helpers, adoption agency handling, ruby/list/form and
select cases, frameset state, foreign-content edge cases, and parser
host callbacks.
Preserve behavior that depends on the C++ DOM integration, including
parser-created custom element reactions, fragment quirks mode, arbitrary
fragment namespaces, template fragment mode, fragment form ownership,
MathML annotation-xml boundaries, contextual fragment scripts, parser
script source positions, document.close() parser state, void-element
insertion, and duplicate attribute tracking.
Add focused tests for the parser edge cases that are easy to regress at
the boundary between the Rust tree builder and the C++ DOM host.
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