fix: atomically replace route metadata during sync

[svelte-triage-bot]

Fixes #17095.

Concurrent syncs could read route_meta_data.json while another process had truncated it for a direct rewrite. This caused intermittent JSON.parse failures.

This change:

• preserves route metadata while stale generated route files are cleaned
• writes metadata to a UUID-named temporary file outside the walked types directory
• atomically renames the complete file into place
• cleans temporary files in a finally block
• adds a deterministic regression test that reproduces the competing-reader parse failure

The unique temporary name avoids collisions, while its location prevents another concurrent sync's types cleanup from removing an in-progress write.

Verification:

• focused regression test
• complete write_types/index.spec.js suite (21 tests)
• pnpm -F @sveltejs/kit test:unit (783 passed)
• pnpm lint
• pnpm check
• pnpm -F @sveltejs/kit prepublishOnly
• 50 rounds of 8 parallel svelte-kit sync processes (0 failures)

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from f87a22d:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/f87a22d80aea6de76eb77dd5cf4727efb9db300a

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17096

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: f87a22d

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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