fix: default engine auto with graphql primary

src/lib/engine.ts

@@ -0,0 +1,15 @@
+export type EngineMode = 'graphql' | 'sweetistics' | 'auto';
+
+export function resolveEngineMode(value?: string): EngineMode {
+  const normalized = (value || 'auto').toLowerCase();
+  if (normalized === 'graphql' || normalized === 'sweetistics' || normalized === 'auto') {
+    return normalized;
+  }
+  return 'auto';
+}
+
+export function shouldUseSweetistics(engine: EngineMode, hasApiKey: boolean): boolean {
+  if (engine === 'sweetistics') return true;
+  if (engine === 'graphql') return false;
+  return false; // auto: GraphQL primary, Sweetistics only as fallback
+}