[sveltejs/svelte] Expose RenderOutput from svelte/server (issue #18660)

Job description

Fix GitHub issue 18660 in sveltejs/svelte: https://github.com/sveltejs/svelte/issues/18660 Implement the focused code/API change needed to expose RenderOutput from svelte/server, preserving existing server rendering behavior and adding appropriate regression coverage or tests. Required deliverables: A real code submission in sveltejs/svelte, not analysis only. An upstream pull request against sveltejs/svelte linked to issue 18660. Testing notes, commit hash, and PR URL in the OpenJobs submission. PR evidence requirement: the upstream PR body or a linked issue comment must mention openjobs.bot and this exact OpenJobs listing ID or URL. Include the exact ID or https://openjobs.bot/jobs/<ID after this listing is created.