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>

tests/pedagogy/test_reward_rules.gd

@@ -0,0 +1,34 @@
+extends GutTest
+## Tests de RewardRules : deblocage des objets selon le total d'etoiles ou le
+## meilleur score d'une mission.
+
+var _catalog: Array[Dictionary] = [
+    {"item_id": "item_total_3", "unlock": {"type": "total_stars", "count": 3}},
+    {"item_id": "item_total_6", "unlock": {"type": "total_stars", "count": 6}},
+    {
+        "item_id": "item_trophy",
+        "unlock": {"type": "mission_stars", "count": 3, "mission_id": "mission_a"}
+    },
+]
+
+
+func test_nothing_unlocks_with_no_stars() -> void:
+    assert_eq(RewardRules.newly_unlocked(_catalog, 0, {}, []), [])
+
+
+func test_total_stars_threshold_unlocks_items_reached() -> void:
+    assert_eq(RewardRules.newly_unlocked(_catalog, 3, {}, []), ["item_total_3"])
+    assert_eq(RewardRules.newly_unlocked(_catalog, 7, {}, []), ["item_total_3", "item_total_6"])
+
+
+func test_owned_items_are_not_unlocked_again() -> void:
+    assert_eq(RewardRules.newly_unlocked(_catalog, 7, {}, ["item_total_3"]), ["item_total_6"])
+
+
+func test_mission_trophy_needs_the_best_score_on_that_mission() -> void:
+    var two_stars := {"mission_a": {"best_stars": 2}}
+    var three_stars := {"mission_a": {"best_stars": 3}}
+    assert_eq(RewardRules.newly_unlocked(_catalog, 0, two_stars, []), [])
+    assert_eq(RewardRules.newly_unlocked(_catalog, 0, three_stars, []), ["item_trophy"])
+    var other_mission := {"mission_b": {"best_stars": 3}}
+    assert_eq(RewardRules.newly_unlocked(_catalog, 0, other_mission, []), [])