Add the playable scene loop: boot through mission to reward

Wires every autoload up to a real UI: boot -> profile select/create
-> world map (three mission markers, positioned from data, not
hardcoded pixels) -> mission intro (narrative from data/missions/,
a new data type with its own schema) -> rule panel -> exercise host
-> one of the three exercise type scenes -> feedback panel -> reward
screen -> back to the map.

exercise_host is the only piece with real orchestration logic: picks
exercises via ExerciseEngine (now fully implemented, validating at
load like make validate-data does), validates answers through the
pure AnswerValidator/DictationAnalyzer, escalates aid over 3 attempts
via AttemptFlow, feeds DifficultyManager so adaptive difficulty is
actually wired in rather than dead code, and records every attempt
through ProgressManager. Every exercise-type scene (multiple_choice,
sentence_correction, dictation) only emits its answer by signal, per
the architecture invariant.

All child-facing text now comes from data/localization/fr.json via
UIText, not hardcoded in scripts or scenes. Getting that right
required extending SchemaValidator to support "additionalProperties"
as a schema (dynamic keys), which also closed a real gap: save.skills
was previously unvalidated against SkillState's actual shape.

16 new headless tests, including two integration tests that instance
exercise_host.tscn directly and drive it by emitting child-scene
signals, stopping short of the final scene transition (which would
tear down the test runner's own tree). Everything that needs a
screen -- layout at different sizes, touch, virtual keyboard, actual
audio playback (no .ogg files exist yet) -- is listed in the new
docs/testing/manual-checklist.md instead of claimed as verified.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scenes/world/mission_intro.tscn

@@ -0,0 +1,59 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="Script" path="res://scenes/world/mission_intro.gd" id="1"]
+[ext_resource type="PackedScene" path="res://scenes/characters/guide_owl.tscn" id="2"]
+
+[node name="MissionIntro" 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 = 32
+theme_override_constants/margin_top = 32
+theme_override_constants/margin_right = 32
+theme_override_constants/margin_bottom = 32
+
+[node name="VBox" type="VBoxContainer" parent="Margin"]
+layout_mode = 2
+theme_override_constants/separation = 16
+
+[node name="TitleLabel" type="Label" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+theme_override_font_sizes/font_size = 32
+
+[node name="NarrativeLabel" type="Label" parent="Margin/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+autowrap_mode = 2
+size_flags_vertical = 3
+
+[node name="Guide" type="Control" parent="Margin/VBox" instance=ExtResource("2")]
+unique_name_in_owner = true
+layout_mode = 2
+
+[node name="Buttons" type="HBoxContainer" parent="Margin/VBox"]
+layout_mode = 2
+theme_override_constants/separation = 16
+
+[node name="BackButton" type="Button" parent="Margin/VBox/Buttons"]
+unique_name_in_owner = true
+layout_mode = 2
+custom_minimum_size = Vector2(0, 56)
+
+[node name="StartButton" type="Button" parent="Margin/VBox/Buttons"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+custom_minimum_size = Vector2(0, 56)