Replace the C++ speculative HTML parser token walk with the Rust
preload scanner. Keep URL resolution, duplicate suppression, and fetch
issuance in C++ so the scanner only emits base href updates and fetch
candidates.
Use the scanner callback result to stop iteration when the speculative
parser has been stopped.
Update parser comments that still described speculative mock element
production.
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.
When the regular HTML parser is blocked on an external script, the
speculative parser scans ahead and pre-fetches discoverable
sub-resources. Previously those fetches were tracked only in the
parser's own URL list and never registered in the document's preload
map, so when the regular parser later reached each element fetch()'s
consume_a_preloaded_resource() lookup found nothing and issued a
duplicate request — every parser-blocked sub-resource was fetched
twice.
issue_speculative_fetch now creates a PreloadEntry, registers it
under create_a_preload_key(request) in the document's preload map,
and supplies a processResponseConsumeBody callback that populates
the entry. The map insertion happens after fetch() starts because
fetch() runs consume_a_preloaded_resource() synchronously, so
registering the entry beforehand would short-circuit the
speculative fetch itself.
The body-handling steps (1, 2, 5 of the preload algorithm's
processResponseConsumeBody) are factored into a shared
deliver_preload_response helper used by both the speculative parser
and HTMLLinkElement::preload.
When the HTML parser blocks on a synchronous external script, run a
separate tokenizer over the unparsed input and issue speculative fetches
for the resources it finds (script src, link rel=stylesheet|preload, img
src), with <base href> tracking and template/foreign-content skipping.
Also fills in the previously-stubbed "consume a preloaded resource"
algorithm and the document's "map of preloaded resources", so that
<link rel="preload"> followed by a matching consumer deduplicates to
a single fetch.
