Svelte 5 with runes reactivity system. Covers components, state
management, and reactive declarations.
USE WHEN: user mentions "Svelte", "runes", "$state", "$derived", "$effect",
"SvelteKit", asks about "Svelte 5 patterns", "reactive runes"
DO NOT USE FOR: Svelte 4 and earlier - use Svelte 4 documentation,
React - use `frontend-react`, Vue - use `vue-composition`, Angular - use `angular`