Delete the old C++ tree-construction implementation and helper classes
that became unused once the Rust parser is unconditional. Remove the C++
stack of open elements, active formatting elements, speculative mock
element, and tree-builder-only token storage.
Keep the C++ parser entry points that still own LibWeb DOM integration,
encoding detection, tokenizer bridging, incremental parsing, and the
speculative parser support used by resource discovery.
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.
