fix: refetch queries after a redirect when the cached query survives the navigation

[Nic-Polumeyv]

Fixes #16444, a regression from #16014 (2.65.1). Root cause analysis is in #16444 (comment).

This moves reset validity into a plain non-reactive field, where batch snapshotting can't hide it. reset() sets a plain #stale flag, #get_promise re-runs the query when the promise it sees is stale or missing, and installing any new promise clears the flag. The ??= write-back goes away with it. On the issue's repro Firefox goes from stale on every submit to correct on every submit, with exactly one refetch per redirect in both engines.

The regression test reproduces the race deterministically in the client unit suite, so it doesn't depend on Firefox. A mounted harness with a persistent layout boundary and a conditionally rendered page boundary, and a gated fetch that keeps the reset batch pending while the page re-renders from a separate batch. It fails on main (the page never refetches) and passes with the fix. Two supporting changes, the client vitest project resolves the browser condition so specs can mount components, and *.spec.svelte fixtures are excluded from the published package.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from fa645e9:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/fa645e9dd2dda34adda8e5efdd50b9a2ee61e019

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16453

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This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

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Latest commit: fa645e9

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@sveltejs/kit	Patch

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