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/schemas/save.schema.json

@@ -8,6 +8,7 @@
     "session_count",
     "unlocked_regions",
     "skills",
+    "mission_results",
     "rewards"
   ],
   "additionalProperties": false,
@@ -48,6 +49,19 @@
         }
       }
     },
+    "mission_results": {
+      "type": "object",
+      "additionalProperties": {
+        "type": "object",
+        "required": ["best_stars", "last_stars", "completions"],
+        "additionalProperties": false,
+        "properties": {
+          "best_stars": { "type": "integer", "minimum": 1, "maximum": 3 },
+          "last_stars": { "type": "integer", "minimum": 1, "maximum": 3 },
+          "completions": { "type": "integer", "minimum": 1 }
+        }
+      }
+    },
     "rewards": {
       "type": "object",
       "required": ["stars", "items"],