fix: refetch route-tracking server data when navigating away from an error page

[Nic-Polumeyv]

After an error page renders, current.route is null (load_root_error_page stores route: null in the navigation state). The next navigation computes

const route_changed = current.route ? route.id !== current.route.id : false;

so the route counts as unchanged no matter where the user goes next. A root layout server load that tracked route is then skipped as fresh, and its data from before the error page renders on the new page. The new test's failing assertion on version-3 shows the layout still claiming the previous route:

Expected: "/b"
Received: "/a"

A missing current.route now counts as changed. On a first-ever navigation the flag is irrelevant, the node has no previous data to reuse anyway, so the only behavior change is the conservative refetch after error pages.

The interaction was never designed. route: null came in #6552 to give beforeNavigate a value meaning "no matched route", and the guard came two months later in #7450 with route tracking itself, mirroring the adjacent url_changed null-guard. Neither discussed the error-page case.

The test lives in the no-ssr app. Its root layout gains a server load returning route.id, the navigation's data request is intercepted with a 500 to produce the error page, and the follow-up navigation must show the new route id. The app's root +layout.server.js was an empty file, shipped by #11354 so its 404-loop test has a server load node in dev; giving it a real load keeps that trigger intact.

Found while reviewing #16380, which mirrors this computation for the error page itself and carries the same one-line fix.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 8a8aca1:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/8a8aca172f1c82395019a83399e67fb41a6c3164

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16381

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Latest commit: 8a8aca1

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[Rich-Harris]

thank you