test: tests unitaires Vitest — format, version-utils, rate-limit

- Installe vitest@4 + @vitest/coverage-v8 (40 tests, 0 échec)
- lib/__tests__/format.test.js        : 14 tests (formatKonstitisyon, formatDate, hasRestrictedChar)
- lib/__tests__/version-utils.test.js : 17 tests (filterVersions par texte/auteur/date, getFilterStats)
- lib/__tests__/rate-limit.test.js    : 9 tests avec fake timers (limite, reset, retryAfter, keys indépendantes)
- vitest.config.mjs : environnement node, imports explicites (pas de globals)
- package.json : scripts test / test:watch / test:coverage + override XO pour les fichiers de test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -3,7 +3,10 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "xo"
+    "lint": "xo",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "@directus/sdk": "^20.3.0",
@@ -28,7 +31,9 @@
     "use-debounce": "^10.0.5"
   },
   "devDependencies": {
+    "@vitest/coverage-v8": "^4.1.4",
     "eslint-config-xo-nextjs": "^6.0.0",
+    "vitest": "^4.1.4",
     "xo": "^0.58.0"
   },
   "xo": {
@@ -49,6 +54,16 @@
       "n/prefer-global/process": "off",
       "comma-dangle": "off",
       "unicorn/prevent-abbreviations": "off"
-    }
+    },
+    "overrides": [
+      {
+        "files": "lib/__tests__/**/*.js",
+        "envs": ["node", "es2020"],
+        "rules": {
+          "camelcase": ["error", {"properties": "never"}],
+          "capitalized-comments": "off"
+        }
+      }
+    ]
   }
 }