feat: nouveau build WebGL last_build + fixes stats et schema Colyseus

- Unity build last_build remplace build_mai
- NetworkSchema.cs: correction types sbyte pour int8 (fix OverflowException Colyseus)
- StatsTracker: envoi periodique toutes les 30s, plus de dependance aux round events
- StatsTracker: cooldown client 6s pour respecter le rate-limit serveur
- StatsPage: correction row.value au lieu de row[activeTab]
- StatsPage: suppression onglet Courses (racesPlayed)
- Backend index.js: logging POST /stats/update
- Scene Tutorial: mise a jour, suppression assets obsoletes (TutorialInfo, physicMaterials)

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

rolld_backend/game/src/index.js

@@ -56,8 +56,12 @@ const gameServer = new Server({
 
     app.post('/stats/update', (req, res) => {
       const parsed = statsUpdateSchema.safeParse(req.body);
-      if (!parsed.success) return res.status(400).json({ error: parsed.error.issues });
+      if (!parsed.success) {
+        console.warn('[Stats] Bad update request:', JSON.stringify(parsed.error.issues));
+        return res.status(400).json({ error: parsed.error.issues });
+      }
       const ok = Stats.update(parsed.data.name, parsed.data.stats);
+      console.log(`[Stats] Update for "${parsed.data.name}": ok=${ok}`, JSON.stringify(parsed.data.stats));
       res.json({ ok });
     });