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>

data/localization/fr.json

@@ -0,0 +1,46 @@
+{
+  "profile_select.title": "Choisis ton profil",
+  "profile_select.new_profile_button": "Nouveau profil",
+  "profile_select.delete_button": "Supprimer",
+  "profile_select.delete_confirm_title": "Supprimer ce profil ?",
+  "profile_select.delete_confirm_message": "Cette action efface pour toujours la progression de ce profil.",
+  "profile_select.delete_confirm_yes": "Oui, supprimer",
+  "profile_select.delete_confirm_no": "Non, annuler",
+  "profile_select.empty_state": "Pas encore de profil : crée le premier !",
+  "profile_create.title": "Crée ton profil",
+  "profile_create.name_label": "Comment t'appelles-tu ?",
+  "profile_create.name_placeholder": "Ton prénom ou ton pseudo",
+  "profile_create.level_label": "Tu es en...",
+  "profile_create.level_ce1": "CE1",
+  "profile_create.level_ce2": "CE2",
+  "profile_create.confirm_button": "Commencer l'aventure",
+  "profile_create.cancel_button": "Retour",
+  "profile_create.name_required_error": "Choisis un prénom avant de continuer.",
+  "world_map.title": "La forêt des accords",
+  "world_map.status_decouverte": "À découvrir",
+  "world_map.status_en_acquisition": "En entraînement",
+  "world_map.status_en_consolidation": "En progrès",
+  "world_map.status_acquise": "Réussie",
+  "world_map.status_a_reviser": "À réviser",
+  "mission_intro.start_button": "Commencer la mission",
+  "mission_intro.back_button": "Retour à la carte",
+  "rule_panel.title": "La règle à retenir",
+  "rule_panel.example_label": "Par exemple :",
+  "rule_panel.continue_button": "C'est parti !",
+  "exercise_host.validate_button": "Valider",
+  "exercise_host.continue_button": "Continuer",
+  "exercise_host.finish_mission_button": "Terminer la mission",
+  "feedback_panel.correct_title": "Bravo !",
+  "feedback_panel.incorrect_title": "Pas tout à fait",
+  "feedback_panel.hint_label": "Indice :",
+  "feedback_panel.explanation_label": "Pourquoi ?",
+  "feedback_panel.correction_label": "La bonne réponse :",
+  "sentence_correction.correction_placeholder": "Corrige ce mot",
+  "dictation.instruction_label": "Écris la phrase que tu entends :",
+  "dictation.input_placeholder": "Écris ici...",
+  "dictation.replay_button": "Réécouter",
+  "dictation.provisional_audio_badge": "Voix provisoire",
+  "reward_screen.title": "Mission réussie !",
+  "reward_screen.stars_label": "Étoiles gagnées :",
+  "reward_screen.continue_button": "Retour à la carte"
+}