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/ui/feedback_panel.gd

@@ -0,0 +1,53 @@
+extends Control
+## Panneau de retour : affiche le resultat d'une tentative (CorrectionResult).
+## Reussite et erreur se distinguent par le texte du titre, jamais par la
+## seule couleur (cahier des charges §11). Instancie par exercise_host.
+
+signal continue_pressed
+
+@onready var _title_label: Label = %TitleLabel
+@onready var _hint_box: VBoxContainer = %HintBox
+@onready var _hint_label: Label = %HintLabel
+@onready var _explanation_box: VBoxContainer = %ExplanationBox
+@onready var _explanation_label: Label = %ExplanationLabel
+@onready var _correction_box: VBoxContainer = %CorrectionBox
+@onready var _correction_label: Label = %CorrectionLabel
+@onready var _continue_button: Button = %ContinueButton
+
+
+func _ready() -> void:
+    _continue_button.text = UIText.get_text("exercise_host.continue_button")
+    _continue_button.pressed.connect(_on_continue_pressed)
+
+
+func show_result(result: CorrectionResult, support_level: AttemptFlow.SupportLevel) -> void:
+    _title_label.text = (
+        UIText.get_text("feedback_panel.correct_title")
+        if result.is_correct
+        else UIText.get_text("feedback_panel.incorrect_title")
+    )
+
+    var show_hint := not result.is_correct and support_level == AttemptFlow.SupportLevel.HINT
+    var show_explanation := (
+        result.is_correct or support_level != AttemptFlow.SupportLevel.HINT
+    )
+    var show_correction := (
+        not result.is_correct and support_level == AttemptFlow.SupportLevel.REVEAL
+    )
+
+    _hint_box.visible = show_hint
+    _hint_label.text = "%s %s" % [UIText.get_text("feedback_panel.hint_label"), result.hint]
+
+    _explanation_box.visible = show_explanation
+    _explanation_label.text = "%s %s" % [
+        UIText.get_text("feedback_panel.explanation_label"), result.explanation
+    ]
+
+    _correction_box.visible = show_correction
+    _correction_label.text = "%s %s" % [
+        UIText.get_text("feedback_panel.correction_label"), ", ".join(result.remediation)
+    ]
+
+
+func _on_continue_pressed() -> void:
+    continue_pressed.emit()