[codex] Avoid state updates in preload tutorial effect

[Sean-Kenneth-Doherty]

Description of Problem

The preloading tutorial measures navigation timing by updating $state inside a $effect. Svelte guidance recommends avoiding state synchronization inside effects, and issue #1366 tracks this tutorial as an example that should be updated.

Closes #1366.

Proposed Solution

Use SvelteKit navigation lifecycle callbacks instead:

• beforeNavigate captures the navigation and start time
• afterNavigate records the completed duration

This keeps the visible tutorial behavior while moving state updates out of $effect.

Additional Information

Validation:

• SHARP_IGNORE_GLOBAL_LIBVIPS=1 mise exec node@24.15.0 -- pnpm install --frozen-lockfile
• mise exec node@24.15.0 -- pnpm prettier --write apps/svelte.dev/content/tutorial/04-advanced-sveltekit/03-link-options/01-preload/+assets/app-a/src/routes/+layout.svelte apps/svelte.dev/content/tutorial/04-advanced-sveltekit/03-link-options/01-preload/+assets/app-b/src/routes/+layout.svelte
• mise exec node@24.15.0 -- pnpm -r package
• mise exec node@24.15.0 -- pnpm --filter svelte.dev lint
• mise exec node@24.15.0 -- pnpm --filter svelte.dev check
• Direct Svelte compiler pass over both changed tutorial layout files
• git diff --check

Current GitHub check note: the only red status is Vercel authorization for the Svelte team account, not a test failure.

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