closes #15927

Calling replaceState or pushState inside a top-level $effect throws Cannot call replaceState(...) before router is initialized in dev, even though everything shallow routing needs already exists at that point.

Twas the release before Svelte 5, and onMount still ran before the router existed, so calling these functions there crashed on root.$set (#11466). #11968 wrapped that crash in a readable dev error, on the reasoning that Svelte 5 had made the problem unreachable since onMount now runs after initial render. That was right about onMount and wrong about $effect, which flushes while the root component is still being created. Everything shallow routing needs already exists in that window, page and the history indexes. So the dev error no longer means what it says.

Two changes.

• the dev guard now checks a router_initialized flag that flips right before the root component is created, once everything shallow routing needs is in place. Genuinely-too-early calls, from module scope or load, throw the same error as before.
• writing the back-navigation test surfaced a second bug that also breaks production. The page.url.href read inside both functions is tracked, so any effect calling them silently depends on page.url, and going back to a pushed entry re-ran the effect, which re-pushed the state it had just left. fix: prevent infinite loop when calling pushState/replaceState in effect #13914 fixed this class for clone_page, this read escaped it. It's untracked now.

started is untouched, it also gates initial fetch handling, navigating and scroll behavior.

An earlier revision targeted main, where the stakes are higher: kit 2's pushState/replaceState still call root.$set for the legacy stores, so the same call crashes production with TypeError: Cannot read properties of undefined (reading '$set'). That part of the fix (falling back to the store while root doesn't exist yet) has no equivalent here since the stores are gone (#15499). If a 2.x patch is wanted I can reopen that version against main.

The new tests fail on version-3 without the fix. In dev both hit the guard, in build the pushState test catches the effect re-run.

Overlaps with #16307 on the PAGE_URL_KEY lines. Whichever merges second, keep resolve_url(url).href and drop the untrack, since resolve_url never had the reactive dependency.

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