fix: include error page assets after SSR render errors

[svelte-triage-bot]

• SSR boundaries render error components outside the loaded branch, so branch-only dependency collection omitted their CSS and fonts.
• Preserve the SSR boundaries introduced in feat: allow error boundaries to catch errors on the server #15308 and owning-depth selection from fix: render the nearest +error.svelte at the depth it occupies when an error is thrown during rendering #16526; retain each error component's node metadata without changing client selection or import-failure fallback behavior.
• Collect dependencies only when an error component is invoked, using unbound argument/return forwarding. Reject eagerly including every possible error page's assets, which would ship unused global CSS on successful responses; leave hydration/data branches unchanged.
• Reuse the existing stylesheet, inline-style, font, preload, CSP, and asset-path pipeline, preserving no-SSR preloading from fix: link stylesheets and fonts in the head even when SSR is disabled #15718. Cover root/nested errors, throwing error pages, and asset isolation between responses.

Fixes #17406

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 3124647:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/3124647d229989e3025b435ba3b489e2689dc6a1

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17408

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: 3124647

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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