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/game_manager.gd

@@ -1,3 +1,23 @@
 extends Node
-## Cycle de vie de l'application, navigation entre scenes et etat general du jeu.
-## Logique detaillee a partir de l'etape "Scenes" (voir Ordre de construction).
+## Cycle de vie de l'application, navigation entre scenes et etat general du
+## jeu. Porte le contexte transitoire necessaire entre deux changements de
+## scene (ex. la mission choisie sur la carte), puisque Godot libere les
+## noeuds de la scene precedente a chaque changement.
+
+const SCENE_BOOT := "res://scenes/boot/boot.tscn"
+const SCENE_PROFILE_SELECT := "res://scenes/profiles/profile_select.tscn"
+const SCENE_PROFILE_CREATE := "res://scenes/profiles/profile_create.tscn"
+const SCENE_WORLD_MAP := "res://scenes/world/world_map.tscn"
+const SCENE_MISSION_INTRO := "res://scenes/world/mission_intro.tscn"
+const SCENE_EXERCISE_HOST := "res://scenes/exercises/exercise_host.tscn"
+const SCENE_REWARD := "res://scenes/ui/reward_screen.tscn"
+
+## Mission choisie sur la carte, lue par mission_intro puis exercise_host et
+## reward_screen. Remise a vide par reward_screen au retour sur la carte.
+var pending_mission_id: String = ""
+
+
+func goto(scene_path: String) -> void:
+    # Differe le changement de scene : l'appeler depuis le _ready() de la
+    # scene en cours d'ajout a l'arbre echoue sinon ("Parent node is busy").
+    get_tree().change_scene_to_file.call_deferred(scene_path)