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>

scenes/exercises/exercise_host.gd

@@ -23,6 +23,8 @@ var _current_difficulty := 1
 var _attempt_number := 1
 var _completed_count := 0
 var _last_exercise_id := ""
+var _outcomes: Array[String] = []
+var _pending_outcome := ""
 
 @onready var _exercise_container: Control = %ExerciseContainer
 @onready var _overlay_container: Control = %OverlayContainer
@@ -43,6 +45,10 @@ func get_completed_count() -> int:
     return _completed_count
 
 
+func get_outcomes() -> Array[String]:
+    return _outcomes
+
+
 func _ready() -> void:
     _mission = ExerciseEngine.get_mission(GameManager.pending_mission_id)
     _rule = ExerciseEngine.get_rule(_mission["rule_id"])
@@ -126,6 +132,15 @@ func _handle_result(result: CorrectionResult) -> void:
         support_level = AttemptFlow.support_level_for_attempt(_attempt_number)
         is_terminal = AttemptFlow.should_end_exercise(_attempt_number)
 
+    if result.is_correct:
+        _pending_outcome = (
+            MissionScoring.OUTCOME_FIRST_TRY
+            if _attempt_number == 1
+            else MissionScoring.OUTCOME_WITH_HELP
+        )
+    elif is_terminal:
+        _pending_outcome = MissionScoring.OUTCOME_REVEALED
+
     ProgressManager.record_attempt(
         _skill_id, result.is_correct, _attempt_number > 1, _mission["exercise_type"]
     )
@@ -154,8 +169,10 @@ func _on_feedback_continued(feedback: Control, is_terminal: bool) -> void:
     )
     _last_exercise_id = _current_exercise.get("id", "")
     _completed_count += 1
+    _outcomes.append(_pending_outcome)
 
     if _completed_count >= MISSION_LENGTH:
+        GameManager.last_outcomes = _outcomes.duplicate()
         GameManager.goto(GameManager.SCENE_REWARD)
     else:
         _load_next_exercise()