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/profiles/profile_create.gd

@@ -0,0 +1,48 @@
+extends Control
+## Creation d'un profil enfant : prenom/pseudo et niveau scolaire. Pas
+## d'avatar ni d'element cosmetique : hors perimetre de la premiere version
+## jouable (voir prompt de lancement, Objectif).
+
+@onready var _title_label: Label = %TitleLabel
+@onready var _name_label: Label = %NameLabel
+@onready var _name_edit: LineEdit = %NameEdit
+@onready var _level_label: Label = %LevelLabel
+@onready var _ce1_button: Button = %CE1Button
+@onready var _ce2_button: Button = %CE2Button
+@onready var _error_label: Label = %ErrorLabel
+@onready var _confirm_button: Button = %ConfirmButton
+@onready var _cancel_button: Button = %CancelButton
+
+
+func _ready() -> void:
+    _title_label.text = UIText.get_text("profile_create.title")
+    _name_label.text = UIText.get_text("profile_create.name_label")
+    _name_edit.placeholder_text = UIText.get_text("profile_create.name_placeholder")
+    _level_label.text = UIText.get_text("profile_create.level_label")
+    _ce1_button.text = UIText.get_text("profile_create.level_ce1")
+    _ce2_button.text = UIText.get_text("profile_create.level_ce2")
+    _error_label.text = UIText.get_text("profile_create.name_required_error")
+    _error_label.visible = false
+    _confirm_button.text = UIText.get_text("profile_create.confirm_button")
+    _cancel_button.text = UIText.get_text("profile_create.cancel_button")
+
+    _ce1_button.button_pressed = true
+
+    _confirm_button.pressed.connect(_on_confirm_pressed)
+    _cancel_button.pressed.connect(_on_cancel_pressed)
+
+
+func _on_confirm_pressed() -> void:
+    var display_name := _name_edit.text.strip_edges()
+    if display_name.is_empty():
+        _error_label.visible = true
+        return
+
+    var school_level := "CE1" if _ce1_button.button_pressed else "CE2"
+    var profile_id := ProfileManager.create_profile(display_name, school_level)
+    if ProfileManager.select_profile(profile_id):
+        GameManager.goto(GameManager.SCENE_WORLD_MAP)
+
+
+func _on_cancel_pressed() -> void:
+    GameManager.goto(GameManager.SCENE_PROFILE_SELECT)