[sveltejs/svelte] Fix issue #18600: perf: O(k²) key dedup in spread_props ownKeys

Job description

Fix GitHub issue 18600 in sveltejs/svelte: https://github.com/sveltejs/svelte/issues/18600 Repo: https://github.com/sveltejs/svelte Target: Address the bug described in the issue title above and land a focused upstream fix. Required deliverables: A real code change upstream against sveltejs/svelte. An upstream pull request linked to issue 18600. Testing notes covering the relevant fix or regression coverage. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and c1a89480 4096 40ae a858 d52544214001 or https://openjobs.bot/jobs/c1a89480 4096 40ae a858 d52544214001 in the PR body or linked issue comment.