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>

autoload/game_manager.gd

@@ -11,11 +11,16 @@ const SCENE_WORLD_MAP := "res://scenes/world/world_map.tscn"
 const SCENE_MISSION_INTRO := "res://scenes/world/mission_intro.tscn"
 const SCENE_EXERCISE_HOST := "res://scenes/exercises/exercise_host.tscn"
 const SCENE_REWARD := "res://scenes/ui/reward_screen.tscn"
+const SCENE_COLLECTION := "res://scenes/ui/collection.tscn"
 
 ## Mission choisie sur la carte, lue par mission_intro puis exercise_host et
 ## reward_screen. Remise a vide par reward_screen au retour sur la carte.
 var pending_mission_id: String = ""
 
+## Resultat de chaque exercice de la mission qui vient de se terminer
+## (MissionScoring.OUTCOME_*), transmis par exercise_host a reward_screen.
+var last_outcomes: Array[String] = []
+
 
 func goto(scene_path: String) -> void:
     # Differe le changement de scene : l'appeler depuis le _ready() de la