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"
+}

data/rewards/item_cle_argentee.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_cle_argentee",
+  "title": "Clé argentée",
+  "description": "Elle ouvre la porte d'un secret caché au fond de la forêt.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 30
+  },
+  "review_status": "draft"
+}

data/rewards/item_couronne_feuilles.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_couronne_feuilles",
+  "title": "Couronne de feuilles",
+  "description": "Tressée avec les feuilles de la forêt des accords.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 22
+  },
+  "review_status": "draft"
+}

data/rewards/item_cristal_echo.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_cristal_echo",
+  "title": "Cristal d'écho",
+  "description": "Il répète en chantant toutes les phrases bien écrites.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 45
+  },
+  "review_status": "draft"
+}

data/rewards/item_graine_magique.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_graine_magique",
+  "title": "Graine magique",
+  "description": "Une petite graine qui brille. Elle attend la bonne forêt pour pousser.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 3
+  },
+  "review_status": "draft"
+}

data/rewards/item_lanterne_foret.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_lanterne_foret",
+  "title": "Lanterne de la forêt",
+  "description": "Elle éclaire les chemins quand la nuit tombe sur les mots.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 10
+  },
+  "review_status": "draft"
+}

data/rewards/item_plume_doree.json

@@ -0,0 +1,10 @@
+{
+  "item_id": "item_plume_doree",
+  "title": "Plume dorée",
+  "description": "Une plume tombée de l'aile de Plume, le hibou guide.",
+  "unlock": {
+    "type": "total_stars",
+    "count": 6
+  },
+  "review_status": "draft"
+}

data/rewards/item_trophee_clairiere.json

@@ -0,0 +1,11 @@
+{
+  "item_id": "item_trophee_clairiere",
+  "title": "Trophée de la clairière",
+  "description": "L'écho de la clairière chante ton nom : tu as tout réussi sans aide.",
+  "unlock": {
+    "type": "mission_stars",
+    "count": 3,
+    "mission_id": "mission_03_clairiere_echo"
+  },
+  "review_status": "draft"
+}

data/rewards/item_trophee_pont.json

@@ -0,0 +1,11 @@
+{
+  "item_id": "item_trophee_pont",
+  "title": "Trophée du pont",
+  "description": "Le pont des phrases brille grâce à toi : tu as tout réussi sans aide.",
+  "unlock": {
+    "type": "mission_stars",
+    "count": 3,
+    "mission_id": "mission_02_pont_des_phrases"
+  },
+  "review_status": "draft"
+}

data/rewards/item_trophee_pre.json

@@ -0,0 +1,11 @@
+{
+  "item_id": "item_trophee_pre",
+  "title": "Trophée du pré",
+  "description": "Les lutins t'offrent ce trophée : tu as tout réussi sans aide au pré des petits mots.",
+  "unlock": {
+    "type": "mission_stars",
+    "count": 3,
+    "mission_id": "mission_01_pre_des_mots"
+  },
+  "review_status": "draft"
+}