Svelte 5 runes ($state, $derived, $effect) only run through the
compiler in .svelte and .svelte.ts/js files. A plain .ts file leaves
$state(...) as a literal call at runtime, causing
"ReferenceError: $state is not defined" and a blank page.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>