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/exercises/multiple_choice.gd

@@ -0,0 +1,30 @@
+extends Control
+## Exercice a choix multiple. Scene autonome : emet la reponse par signal,
+## ne touche ni a la sauvegarde ni a la progression (invariant d'architecture,
+## CLAUDE.md).
+
+signal answer_submitted(selected_index: int)
+
+@onready var _instruction_label: Label = %InstructionLabel
+@onready var _question_label: Label = %QuestionLabel
+@onready var _answers_box: VBoxContainer = %AnswersBox
+
+
+func load_exercise(exercise: Dictionary) -> void:
+    _instruction_label.text = exercise["instruction"]
+    _question_label.text = exercise["question"]
+
+    for child in _answers_box.get_children():
+        child.queue_free()
+
+    var answers: Array = exercise["answers"]
+    for i in answers.size():
+        var button := Button.new()
+        button.text = answers[i]
+        button.custom_minimum_size = Vector2(0, 56)
+        button.pressed.connect(_on_answer_pressed.bind(i))
+        _answers_box.add_child(button)
+
+
+func _on_answer_pressed(index: int) -> void:
+    answer_submitted.emit(index)