test: add characterization tests for lib layer

Cover classic-mode comparison rules, game-engine state machine and
localStorage persistence, and champion-data search/seeding/loading with
Vitest. Storage keys are asserted literally so returning players' saved
games and stats cannot silently break.

Node environment with a hand-rolled localStorage stub; fixtures are four
hand-written champions so the weekly data sync cannot destabilise the
suite.

web/package.json

@@ -7,7 +7,9 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "next": "16.2.10",
@@ -18,6 +20,7 @@
     "@tailwindcss/postcss": "^4.3.3",
     "eslint": "^9",
     "eslint-config-next": "16.2.10",
-    "tailwindcss": "^4.3.3"
+    "tailwindcss": "^4.3.3",
+    "vitest": "^4.1.10"
   }
 }