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>

autoload/audio_manager.gd

@@ -1,3 +1,26 @@
 extends Node
-## Lecture des voix, dictees, musiques et effets sonores, y compris l'audio provisoire des dictees.
-## Logique detaillee a partir de l'etape "Scenes" (voir Ordre de construction).
+## Lecture des voix, dictees, musiques et effets sonores, y compris l'audio
+## provisoire des dictees. Aucun fichier audio n'est encore fourni (etape
+## "Graphismes et audio provisoires", a venir) : play_dictation se degrade
+## silencieusement (avertissement journalise) si le fichier est absent,
+## plutot que de faire planter le jeu.
+
+const DICTATION_AUDIO_DIR := "res://assets/audio/dictations"
+
+var _player: AudioStreamPlayer
+
+
+func _ready() -> void:
+    _player = AudioStreamPlayer.new()
+    add_child(_player)
+
+
+func play_dictation(audio_id: String) -> void:
+    if audio_id.is_empty():
+        return
+    var path := "%s/%s.ogg" % [DICTATION_AUDIO_DIR, audio_id]
+    if not ResourceLoader.exists(path):
+        push_warning("Audio de dictee introuvable (pas encore genere) : %s" % path)
+        return
+    _player.stream = load(path)
+    _player.play()