Add mission scoring, rewards collection and save schema v2

Phase "score et récompenses" of the post-0.1.0 plan (user feedback:
wants a more game-like experience with scoring and rewards).

- MissionScoring: 1-3 stars from how each exercise was solved (never
  speed; a finished mission always gives at least one star).
- Save schema v2: mission_results (best/last stars, completions) with a
  v1->v2 migration that keeps existing stars and items.
- Total only grows by the improvement over a mission's best score.
- Data-driven collection (data/rewards, reward_item schema, validated
  incl. mission cross-check) unlocked by RewardRules.
- Reward screen, collection screen, stars on the world map.
- Version 0.2.0.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/profiles/save_migration.gd

@@ -1,10 +1,11 @@
 class_name SaveMigration
 extends RefCounted
-## Migration des sauvegardes entre versions de schema. Code pur. Une seule
-## version existe pour l'instant (1) ; chaque montee de version future ajoute
-## une etape a la chaine ci-dessous, sans jamais reecrire les etapes passees.
+## Migration des sauvegardes entre versions de schema. Code pur. Chaque
+## montee de version ajoute une etape a la chaine ci-dessous, sans jamais
+## reecrire les etapes passees.
+## Version 2 : ajout de mission_results (meilleur score par mission).
 
-const CURRENT_SCHEMA_VERSION := 1
+const CURRENT_SCHEMA_VERSION := 2
 
 
 ## Retourne un dictionnaire vide si la version est absente, invalide, ou plus
@@ -21,10 +22,17 @@ static func migrate(data: Dictionary) -> Dictionary:
         return {}
 
     var migrated: Dictionary = data.duplicate(true)
-    # Chaine de montees de version, a completer quand CURRENT_SCHEMA_VERSION
-    # augmentera :
-    # if version_int == 1:
-    #     migrated = _migrate_v1_to_v2(migrated)
-    #     version_int = 2
+    if version_int == 1:
+        migrated = _migrate_v1_to_v2(migrated)
+        version_int = 2
     migrated["schema_version"] = CURRENT_SCHEMA_VERSION
     return migrated
+
+
+## Les etoiles deja gagnees (rewards.stars) sont conservees telles quelles ;
+## le detail par mission n'existait pas, il repart vide.
+static func _migrate_v1_to_v2(data: Dictionary) -> Dictionary:
+    var migrated: Dictionary = data.duplicate(true)
+    if not migrated.has("mission_results"):
+        migrated["mission_results"] = {}
+    return migrated