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/reward_rules.gd

@@ -0,0 +1,34 @@
+class_name RewardRules
+extends RefCounted
+## Regles de deblocage des objets de la collection, a partir du catalogue
+## valide de data/rewards/. Code pur : renvoie ce qui vient d'etre debloque,
+## ProgressManager se charge de l'enregistrer.
+
+
+## catalog : liste d'objets conformes a reward_item.schema.json.
+## best_stars_by_mission : mission_id -> {"best_stars": int, ...}.
+## owned : item_id deja possedes (jamais redebloques).
+static func newly_unlocked(
+    catalog: Array[Dictionary], total_stars: int, best_stars_by_mission: Dictionary, owned: Array
+) -> Array[String]:
+    var unlocked: Array[String] = []
+    for item in catalog:
+        var item_id: String = item["item_id"]
+        if owned.has(item_id) or unlocked.has(item_id):
+            continue
+        if _is_unlocked(item["unlock"], total_stars, best_stars_by_mission):
+            unlocked.append(item_id)
+    return unlocked
+
+
+static func _is_unlocked(
+    unlock: Dictionary, total_stars: int, best_stars_by_mission: Dictionary
+) -> bool:
+    var count: int = int(unlock["count"])
+    match unlock["type"]:
+        "total_stars":
+            return total_stars >= count
+        "mission_stars":
+            var result: Dictionary = best_stars_by_mission.get(unlock.get("mission_id", ""), {})
+            return int(result.get("best_stars", 0)) >= count
+    return false