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/pedagogy/mission_scoring.gd

@@ -0,0 +1,29 @@
+class_name MissionScoring
+extends RefCounted
+## Score d'une mission en etoiles (1 a 3), calcule a partir de ce qui s'est
+## passe sur chaque exercice. Code pur. Le score valorise la reussite sans
+## aide, jamais la vitesse ni la duree (cahier des charges §2) ; ce n'est
+## pas une note scolaire (§9). Une mission terminee rapporte toujours au
+## moins une etoile : l'erreur n'est jamais punie.
+
+const OUTCOME_FIRST_TRY := "first_try"
+const OUTCOME_WITH_HELP := "with_help"
+const OUTCOME_REVEALED := "revealed"
+
+const MIN_STARS := 1
+const MAX_STARS := 3
+
+
+## 3 etoiles : tous les exercices reussis du premier coup.
+## 2 etoiles : tous reussis, au moins un apres un indice ou une explication.
+## 1 etoile : au moins une reponse a ete montree (mission terminee quand meme).
+static func stars_for(outcomes: Array[String]) -> int:
+    if outcomes.is_empty():
+        return MIN_STARS
+    var all_first_try := true
+    for outcome in outcomes:
+        if outcome == OUTCOME_REVEALED:
+            return 1
+        if outcome != OUTCOME_FIRST_TRY:
+            all_first_try = false
+    return MAX_STARS if all_first_try else 2