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.
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.
