Svelte Guideline: Reactivity — Use $state in Svelte 5. Description: Runes for explicit reactivity. Do: let count = $state(0). Don't: Implicit reactivity in Svelte 5. Good Example: let count = $state(0). Bad Example: let count = 0 (Svelte 5). Severity: Medium. Docs: https://svelte.dev/blog/runes.