feat(games): add GamePlugin interface and registry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/games/registry.ts

@@ -0,0 +1,18 @@
+import type { GamePlugin } from "./types";
+
+const games = new Map<string, GamePlugin>();
+
+export const gameRegistry = {
+    register(plugin: GamePlugin) {
+        if (games.has(plugin.slug)) {
+            throw new Error(`Game "${plugin.slug}" is already registered`);
+        }
+        games.set(plugin.slug, plugin);
+    },
+    get(slug: string): GamePlugin | undefined {
+        return games.get(slug);
+    },
+    list(): GamePlugin[] {
+        return Array.from(games.values());
+    },
+};

shared/games/types.ts

@@ -0,0 +1,23 @@
+export interface GamePlugin<TState = unknown, TAction = unknown> {
+    slug: string;
+    name: string;
+    minPlayers: number;
+    maxPlayers: number;
+
+    createInitialState(players: string[]): TState;
+    handleAction(state: TState, action: TAction, playerId: string): GameResult<TState>;
+    getPlayerView(state: TState, playerId: string): unknown;
+    getSpectatorView(state: TState): unknown;
+
+    isGameOver?(state: TState): GameOverResult | null;
+    onPlayerDisconnect?(state: TState, playerId: string): TState;
+}
+
+export type GameResult<TState> =
+    | { ok: true; state: TState }
+    | { ok: false; error: string };
+
+export type GameOverResult = {
+    winner: string | null;
+    reason: string;
+};