Closes #8032

What

• Document SvelteKit backend instrumentation for fullstack setups.
• Add a hooks.server.ts example using @highlight-run/node + H.runWithHeaders to connect backend traces/errors to the originating frontend session.

Bounty

• /claim #8032

Verification

• yarn workspace @highlight-run/next build
• cd highlight.io && yarn build (passes; warnings are pre-existing)