fix: resolve server-routed CSS against its route module

[svelte-triage-bot]

• Server-resolved CSS paths were relative to __route.js, but load_css interpreted them against the departing document, causing incorrect requests and duplicate stylesheet links.
• Resolve dependencies against import.meta.url before loading CSS. Keep the portable paths introduced by #17056 rather than reverting to root-absolute URLs and breaking prefix-stripping proxies.
• Preserve the dedicated CSS loading from #13498 and SSR stylesheet deduplication from #15014; cover navigation, code/data preloading, and returning from deeper routes.
• The shared generator covers runtime and prerendered pathname/route-ID modules, including those added by #16576. HTML path helpers and client-side route resolution remain unchanged.

Fixes #17412

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from df04593:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/df045939207742d625980f02bb6553d43cde3682

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17414

Note

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

[changeset-bot]

🦋 Changeset detected

Latest commit: df04593

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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