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/world/world_map.gd

@@ -0,0 +1,56 @@
+extends Control
+## Carte a trois lieux (cahier des charges §15). La position de chaque mission
+## vient de data/missions/*.json (fractions 0..1 de la zone de carte), jamais
+## d'une position fixe en dur dans le script.
+
+const MARKER_HALF_WIDTH := 80.0
+const MARKER_HALF_HEIGHT := 36.0
+
+@onready var _title_label: Label = %TitleLabel
+@onready var _map_area: Control = %MapArea
+
+
+func _ready() -> void:
+    _title_label.text = UIText.get_text("world_map.title")
+    _populate_missions()
+
+
+func _populate_missions() -> void:
+    for child in _map_area.get_children():
+        child.queue_free()
+
+    for mission: Dictionary in ExerciseEngine.get_missions():
+        _map_area.add_child(_build_marker(mission))
+
+
+func _build_marker(mission: Dictionary) -> Control:
+    var button := Button.new()
+    button.text = "%s\n%s" % [mission["title"], _status_label_for(mission)]
+
+    var map_position: Dictionary = mission["map_position"]
+    var x: float = map_position["x"]
+    var y: float = map_position["y"]
+    button.anchor_left = x
+    button.anchor_right = x
+    button.anchor_top = y
+    button.anchor_bottom = y
+    button.offset_left = -MARKER_HALF_WIDTH
+    button.offset_right = MARKER_HALF_WIDTH
+    button.offset_top = -MARKER_HALF_HEIGHT
+    button.offset_bottom = MARKER_HALF_HEIGHT
+
+    button.pressed.connect(_on_mission_pressed.bind(mission["mission_id"]))
+    return button
+
+
+func _status_label_for(mission: Dictionary) -> String:
+    var rule: Dictionary = ExerciseEngine.get_rule(mission["rule_id"])
+    var skill_id: String = rule.get("skill", "")
+    var skill_state := ProgressManager.get_skill_state(skill_id)
+    var status: String = skill_state["status"]
+    return UIText.get_text("world_map.status_%s" % status)
+
+
+func _on_mission_pressed(mission_id: String) -> void:
+    GameManager.pending_mission_id = mission_id
+    GameManager.goto(GameManager.SCENE_MISSION_INTRO)