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/data/semantic_checks.gd

@@ -50,3 +50,21 @@ static func check_dictation(data: Dictionary, path: String) -> Array[String]:
             )
         )
     return errors
+
+
+static func check_reward_item(data: Dictionary, path: String) -> Array[String]:
+    var errors: Array[String] = []
+    var unlock: Dictionary = data.get("unlock", {})
+    var count: int = int(unlock.get("count", 0))
+    match unlock.get("type", ""):
+        "total_stars":
+            if unlock.has("mission_id"):
+                errors.append("%s : total_stars ne prend pas de mission_id" % path)
+        "mission_stars":
+            if not unlock.has("mission_id"):
+                errors.append("%s : mission_stars exige un mission_id" % path)
+            if count > 3:
+                errors.append(
+                    "%s : une mission rapporte au plus 3 etoiles (count=%d)" % [path, count]
+                )
+    return errors