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/exercises/sentence_correction.gd

@@ -0,0 +1,59 @@
+extends Control
+## Exercice de correction de phrase : toucher le mot fautif puis le corriger
+## (interaction decidee dans docs/specs/decisions.md, point 3). Scene
+## autonome : emet la reponse par signal, ne touche ni a la sauvegarde ni a
+## la progression.
+
+signal answer_submitted(selected_indices: Array, corrections: Dictionary)
+
+var _correction_edits: Dictionary = {}
+
+@onready var _instruction_label: Label = %InstructionLabel
+@onready var _tokens_box: HFlowContainer = %TokensBox
+@onready var _corrections_box: VBoxContainer = %CorrectionsBox
+@onready var _validate_button: Button = %ValidateButton
+
+
+func _ready() -> void:
+    _validate_button.text = UIText.get_text("exercise_host.validate_button")
+    _validate_button.pressed.connect(_on_validate_pressed)
+
+
+func load_exercise(exercise: Dictionary) -> void:
+    _instruction_label.text = exercise["instruction"]
+
+    for child in _tokens_box.get_children():
+        child.queue_free()
+    for child in _corrections_box.get_children():
+        child.queue_free()
+    _correction_edits.clear()
+
+    var tokens: Array = exercise["tokens"]
+    for i in tokens.size():
+        var button := Button.new()
+        button.text = tokens[i]["text"]
+        button.toggle_mode = true
+        button.toggled.connect(_on_token_toggled.bind(i))
+        _tokens_box.add_child(button)
+
+
+func _on_token_toggled(pressed: bool, index: int) -> void:
+    if pressed:
+        if not _correction_edits.has(index):
+            var edit := LineEdit.new()
+            edit.placeholder_text = UIText.get_text("sentence_correction.correction_placeholder")
+            _corrections_box.add_child(edit)
+            _correction_edits[index] = edit
+    elif _correction_edits.has(index):
+        var edit: LineEdit = _correction_edits[index]
+        edit.queue_free()
+        _correction_edits.erase(index)
+
+
+func _on_validate_pressed() -> void:
+    var selected_indices: Array = []
+    var corrections: Dictionary = {}
+    for index: int in _correction_edits.keys():
+        selected_indices.append(index)
+        corrections[index] = (_correction_edits[index] as LineEdit).text.strip_edges()
+    answer_submitted.emit(selected_indices, corrections)