feat: Sprint 3 — Prestige Loop endless

- Migration saves: saveVersion pattern + migrateSave lazy (v1→v2)
- Formule ADN rebalancée: log10 + clamp min 1 + cap bonus ×4
- Prestige Experience: modal fullscreen, preview ADN, stats run, best run
- Arbre V2: 25 nœuds, 3 capstones, post-capstones repeatables (scaling par tranche)
- Convergence évolutif Alpha→Omega (tier system)
- Reset arbre: 1 gratuit/prestige, payant linéaire au-delà
- Milestones prestige: 8 paliers (1→100), cosmétiques exclusifs, bonus gameplay
- balance.ts: constantes centralisées pour playtest
- 136 tests green, 0 regression

Backend/src/controllers/saveControllers.js

@@ -1,4 +1,5 @@
 const tables = require("../tables");
+const { migrateSave } = require("../services/migrateSave");
 
 // --- Anti-cheat validation ---
 
@@ -75,11 +76,14 @@ const load = async (req, res) => {
     }
 
     // game_state est stocké en JSON — MySQL le retourne comme objet si type JSON
-    const gameState =
+    const rawState =
       typeof save.game_state === "string"
         ? JSON.parse(save.game_state)
         : save.game_state;
 
+    // Migrate on load — lazy migration, never touch DB rows directly
+    const gameState = migrateSave(rawState);
+
     return res.status(200).json({
       gameState,
       lastSave: save.last_save,

Backend/src/models/GameSaveManager.js

@@ -15,18 +15,20 @@ class GameSaveManager extends AbstractManager {
 
   async upsert(userId, gameState, metadata) {
     const { lifetimeTadpoles, prestigeCount, playTimeSeconds } = metadata;
+    const saveVersion = gameState.saveVersion ?? 1;
     const gameStateJson = JSON.stringify(gameState);
 
     const [result] = await this.database.query(
-      `INSERT INTO ${this.table} (user_id, game_state, lifetime_tadpoles, prestige_count, play_time_seconds)
-       VALUES (?, ?, ?, ?, ?)
+      `INSERT INTO ${this.table} (user_id, game_state, save_version, lifetime_tadpoles, prestige_count, play_time_seconds)
+       VALUES (?, ?, ?, ?, ?, ?)
        ON DUPLICATE KEY UPDATE
          game_state = VALUES(game_state),
+         save_version = VALUES(save_version),
          lifetime_tadpoles = VALUES(lifetime_tadpoles),
          prestige_count = VALUES(prestige_count),
          play_time_seconds = VALUES(play_time_seconds),
          last_save = CURRENT_TIMESTAMP`,
-      [userId, gameStateJson, lifetimeTadpoles, prestigeCount, playTimeSeconds]
+      [userId, gameStateJson, saveVersion, lifetimeTadpoles, prestigeCount, playTimeSeconds]
     );
 
     return result.affectedRows;

Backend/src/services/migrateSave.js

@@ -0,0 +1,65 @@
+// migrateSave.js — Backend save migration (mirrors Frontend/src/core/migrateSave.ts)
+// Applied on load — lazy migration, never touch DB directly.
+
+const CURRENT_SAVE_VERSION = 2;
+
+// Default evolution tree (Sprint 2 — 18 nodes)
+// Used to merge new nodes into old saves
+const DEFAULT_TREE_IDS = [
+  "ponte_amelioree", "double_ponte", "ponte_frenetique", "auto_ponte",
+  "ponte_critique", "maitre_pondeur",
+  "instinct_gregaire", "symbiose_algale", "courant_profond", "maree_haute",
+  "ecosysteme_mature", "marais_eternel",
+  "memoire_genetique", "adn_renforce", "eveil_rapide", "resilience",
+  "heritage", "transcendance",
+];
+
+/**
+ * Migrate a raw game state to the current version.
+ * Backend only needs structural migration for anti-cheat validation —
+ * the full tree/generator merge happens on the frontend.
+ */
+function migrateSave(raw) {
+  if (!raw || typeof raw !== "object") return raw;
+
+  const version = typeof raw.saveVersion === "number" ? raw.saveVersion : 1;
+  let state = { ...raw };
+
+  if (version < 2) {
+    state = migrateV1toV2(state);
+  }
+
+  return state;
+}
+
+function migrateV1toV2(state) {
+  state.saveVersion = 2;
+
+  // RunStats
+  if (!state.runStats) {
+    state.runStats = {
+      startedAt: state.lastTick || Date.now(),
+      tadpolesProduced: 0,
+      bestRun: null,
+    };
+  }
+
+  // Tree reset fields
+  if (typeof state.freeResetAvailable !== "boolean") {
+    state.freeResetAvailable = true;
+  }
+  if (typeof state.extraResetsUsed !== "number") {
+    state.extraResetsUsed = 0;
+  }
+
+  // Backfill cosmetics
+  if (!state.lastOnline) state.lastOnline = state.lastTick;
+  if (!Array.isArray(state.cosmeticInventory)) state.cosmeticInventory = [];
+  if (!state.cosmeticEquipped || typeof state.cosmeticEquipped !== "object") {
+    state.cosmeticEquipped = {};
+  }
+
+  return state;
+}
+
+module.exports = { migrateSave, CURRENT_SAVE_VERSION };