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/rule_panel.gd

@@ -0,0 +1,29 @@
+extends Control
+## Panneau de regle : affiche l'enonce et l'exemple issus de data/rules/
+## (cycle d'apprentissage §2, etape "Decouvrir la regle"). Instancie par
+## exercise_host, qui ecoute le signal `continued` pour lancer le premier
+## exercice.
+
+signal continued
+
+@onready var _title_label: Label = %TitleLabel
+@onready var _statement_label: Label = %StatementLabel
+@onready var _example_label: Label = %ExampleLabel
+@onready var _continue_button: Button = %ContinueButton
+
+
+func _ready() -> void:
+    _title_label.text = UIText.get_text("rule_panel.title")
+    _continue_button.text = UIText.get_text("rule_panel.continue_button")
+    _continue_button.pressed.connect(_on_continue_pressed)
+
+
+func show_rule(rule: Dictionary) -> void:
+    _statement_label.text = rule.get("statement", "")
+    _example_label.text = "%s %s" % [
+        UIText.get_text("rule_panel.example_label"), rule.get("example", "")
+    ]
+
+
+func _on_continue_pressed() -> void:
+    continued.emit()