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>

data/rewards/item_carte_ancienne.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_carte_ancienne",
+  "title": "Carte ancienne",
+  "description": "Une vieille carte qui montre des lieux que personne n'a encore visités.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 15
+  },
+  "review_status": "draft"
+}