Replay same-document traversals that happen before hydration (#95682)
Fixes a race you can observe if you:
1. Open page A
2. Navigate to page B
3. Cmd+Shift+R while on page B...
4. ...and then navigate Back super fast
In that situation, the router gets confused and shows page B's content
with page A's URL. From then on, going Back and Forward changes the URL
(and tab title) but keeps showing B's content. See the new tests.
## Why This Happens
Navigating from A to B client-side creates B's history entry via
`pushState`. Reloading B replaces that document with B's fresh
server-rendered HTML. Pressing Back at that point fires `popstate` to A,
but nothing listens to it yet. Then JS loads, and we happily hydrate B's
content onto B's HTML despite technically being on A route.
## The Fix
Use Navigation API to detect that we’re on a different entry than we
started. Once the router mounts, replay the missed traversal the same
way the `onPopState` handler would have handled it.
(I originally tried to avoid using Navigation API but wasn’t sure how to
detect this reliably.)
## Test Plan
New tests are red before the fix.
next.js
2ce362f3 - Replay same-document traversals that happen before hydration (#95682)
