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/dictation.gd

@@ -0,0 +1,41 @@
+extends Control
+## Exercice de dictee : ecoute puis ecrit. Scene autonome : emet la reponse
+## par signal, ne touche ni a la sauvegarde ni a la progression. L'audio est
+## provisoire (voir docs/specs/decisions.md, point 8) ; le badge l'indique a
+## l'enfant tant qu'aucune voix definitive n'a remplace le fichier.
+
+signal answer_submitted(submitted_text: String)
+
+var _dictation: Dictionary = {}
+
+@onready var _instruction_label: Label = %InstructionLabel
+@onready var _replay_button: Button = %ReplayButton
+@onready var _provisional_badge: Label = %ProvisionalBadge
+@onready var _text_edit: LineEdit = %TextEdit
+@onready var _validate_button: Button = %ValidateButton
+
+
+func _ready() -> void:
+    _instruction_label.text = UIText.get_text("dictation.instruction_label")
+    _replay_button.text = UIText.get_text("dictation.replay_button")
+    _text_edit.placeholder_text = UIText.get_text("dictation.input_placeholder")
+    _provisional_badge.text = UIText.get_text("dictation.provisional_audio_badge")
+    _validate_button.text = UIText.get_text("exercise_host.validate_button")
+
+    _replay_button.pressed.connect(_on_replay_pressed)
+    _validate_button.pressed.connect(_on_validate_pressed)
+
+
+func load_exercise(dictation: Dictionary) -> void:
+    _dictation = dictation
+    _text_edit.text = ""
+    _provisional_badge.visible = dictation.get("audio_provisional", false)
+    _on_replay_pressed()
+
+
+func _on_replay_pressed() -> void:
+    AudioManager.play_dictation(_dictation.get("audio_id", ""))
+
+
+func _on_validate_pressed() -> void:
+    answer_submitted.emit(_text_edit.text)