Finalize fragment navigations and URL/history updates immediately when
no traversal state is active. Keep the queued same-document finalizer as
the fallback for reentrant traversal work and child navigables whose
nested history is not installed yet.
Share the entry-list portion of same-document navigation finalization
between the fast path and queued fallback, so append and replace
bookkeeping cannot drift.
Preserve unrelated ongoing cross-document navigations when a page starts
a load and then performs a same-document history update in the same
task. This matches Chromium, WebKit, and Gecko: the same-document
update must not cancel the pending real navigation.
The session-history mirror tests now observe synchronous UI updates. A
navigation test covers the pending-load plus pushState race.
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