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/exercises/test_exercise_engine.gd

@@ -29,6 +29,16 @@ func test_missions_are_filtered_by_the_level_of_their_rule() -> void:
     assert_eq(ExerciseEngine.get_missions_for_level("CM1").size(), 0)
 
 
+func test_reward_catalog_is_loaded_and_sorted_by_threshold() -> void:
+    var items := ExerciseEngine.get_reward_items()
+    assert_eq(items.size(), 10)
+    var counts: Array = items.map(func(i: Dictionary) -> int: return i["unlock"]["count"])
+    var sorted_counts := counts.duplicate()
+    sorted_counts.sort()
+    assert_eq(counts, sorted_counts)
+    assert_eq(items[0]["item_id"], "item_graine_magique", "seuil 3, ordre alphabetique")
+
+
 func test_get_mission_returns_the_requested_mission() -> void:
     var mission := ExerciseEngine.get_mission("mission_02_pont_des_phrases")
     assert_eq(mission["exercise_type"], "sentence_correction")

tests/exercises/test_exercise_host_outcomes.gd

@@ -0,0 +1,58 @@
+extends GutTest
+## exercise_host note, pour chaque exercice termine, comment il a ete resolu
+## (du premier coup, avec aide, reponse montree) : c'est ce qui alimente le
+## score de la mission (MissionScoring). Le changement de scene final n'est
+## jamais atteint ici.
+
+var _profile_id: String = ""
+
+
+func before_each() -> void:
+    _profile_id = ProfileManager.create_profile("Test", "CE1")
+    ProfileManager.select_profile(_profile_id)
+    GameManager.pending_mission_id = "mission_01_pre_des_mots"
+
+
+func after_each() -> void:
+    ProfileManager.delete_profile(_profile_id)
+    GameManager.pending_mission_id = ""
+
+
+func _open_host() -> Control:
+    var host: Control = load("res://scenes/exercises/exercise_host.tscn").instantiate()
+    add_child_autofree(host)
+    await wait_process_frames(1)
+    host.get_node("%OverlayContainer").get_child(0).continued.emit()
+    await wait_process_frames(1)
+    return host
+
+
+func _answer(host: Control, correctly: bool) -> void:
+    var exercise: Dictionary = host.get_current_exercise()
+    var correct: int = exercise["correct_answer"]
+    var choice := correct if correctly else (correct + 1) % (exercise["answers"] as Array).size()
+    host.get_node("%ExerciseContainer").get_child(0).answer_submitted.emit(choice)
+    await wait_process_frames(1)
+    var overlay: Control = host.get_node("%OverlayContainer")
+    overlay.get_child(overlay.get_child_count() - 1).get_node("%ContinueButton").pressed.emit()
+    await wait_process_frames(1)
+
+
+func test_correct_first_try_then_found_with_help_then_revealed() -> void:
+    var host: Control = await _open_host()
+
+    await _answer(host, true)
+    await _answer(host, false)
+    await _answer(host, true)
+    await _answer(host, false)
+    await _answer(host, false)
+    await _answer(host, false)
+
+    assert_eq(
+        host.get_outcomes(),
+        [
+            MissionScoring.OUTCOME_FIRST_TRY,
+            MissionScoring.OUTCOME_WITH_HELP,
+            MissionScoring.OUTCOME_REVEALED,
+        ]
+    )

tests/exercises/test_exercise_host_outcomes.gd.uid

@@ -0,0 +1 @@
+uid://dkucbjqnwv2oj