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>

scenes/ui/collection.tscn

@@ -0,0 +1,54 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scenes/ui/collection.gd" id="1"]
+
+[node name="Collection" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+script = ExtResource("1")
+
+[node name="Margin" type="MarginContainer" parent="."]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+theme_override_constants/margin_left = 24
+theme_override_constants/margin_top = 24
+theme_override_constants/margin_right = 24
+theme_override_constants/margin_bottom = 24
+
+[node name="VBox" type="VBoxContainer" parent="Margin"]
+layout_mode = 2
+theme_override_constants/separation = 12
+
+[node name="TitleLabel" type="Label" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+theme_override_font_sizes/font_size = 32
+horizontal_alignment = 1
+
+[node name="ProgressLabel" type="Label" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+horizontal_alignment = 1
+
+[node name="Scroll" type="ScrollContainer" parent="Margin/VBox"]
+layout_mode = 2
+size_flags_vertical = 3
+
+[node name="ItemsBox" type="VBoxContainer" parent="Margin/VBox/Scroll"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+theme_override_constants/separation = 16
+
+[node name="BackButton" type="Button" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+custom_minimum_size = Vector2(0, 56)