Exported $state proxies were confused with Svelte stores by SvelteKit
runtime, causing "s.subscribe is not a function" on /jeu.
Fix: encapsulate all $state fields in a Game class, export singleton.
Components import { game } and access game.state, game.click(), etc.
Class fields are proper $state — no raw proxy exported.
Svelte 5 can't export reassigned $state — use const $state + Object.assign.
All components now import state/actions directly (no gameStore wrapper).
Deep reactivity works: evolutionTree nodes, generators, cosmetics all tracked.