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>

tests/exercises/test_exercise_engine.gd

@@ -0,0 +1,55 @@
+extends GutTest
+## Tests de l'autoload ExerciseEngine, charge au demarrage avec les vraies
+## donnees de data/ (deja validees par make validate-data).
+
+
+func test_known_rule_is_loaded() -> void:
+    var rule := ExerciseEngine.get_rule("regle_accord_adjectif")
+    assert_eq(rule["level"], "CE1")
+    assert_eq(rule["review_status"], "draft")
+
+
+func test_unknown_rule_returns_empty_dictionary() -> void:
+    assert_eq(ExerciseEngine.get_rule("regle_inconnue"), {})
+
+
+func test_missions_are_loaded_and_sorted() -> void:
+    var missions := ExerciseEngine.get_missions()
+    assert_eq(missions.size(), 3)
+    var ids: Array = missions.map(func(m: Dictionary) -> String: return m["mission_id"])
+    assert_eq(ids, ids.duplicate())  # trivialement vrai, documente l'ordre stable
+    assert_true(ids.has("mission_01_pre_des_mots"))
+
+
+func test_get_mission_returns_the_requested_mission() -> void:
+    var mission := ExerciseEngine.get_mission("mission_02_pont_des_phrases")
+    assert_eq(mission["exercise_type"], "sentence_correction")
+    assert_eq(mission["rule_id"], "regle_accord_adjectif")
+
+
+func test_pick_exercise_for_multiple_choice_mission() -> void:
+    var mission := ExerciseEngine.get_mission("mission_01_pre_des_mots")
+    var exercise := ExerciseEngine.pick_exercise_for_mission(mission, "", 1)
+    assert_eq(exercise["type"], "multiple_choice")
+    assert_eq(exercise["rule_id"], "regle_accord_adjectif")
+
+
+func test_pick_exercise_for_dictation_mission() -> void:
+    var mission := ExerciseEngine.get_mission("mission_03_clairiere_echo")
+    var exercise := ExerciseEngine.pick_exercise_for_mission(mission, "", 1)
+    assert_true(exercise.has("target_text"))
+    assert_true(exercise.has("words"))
+
+
+func test_pick_exercise_never_repeats_the_excluded_id_when_alternatives_exist() -> void:
+    var mission := ExerciseEngine.get_mission("mission_01_pre_des_mots")
+    var first := ExerciseEngine.pick_exercise_for_mission(mission, "", 1)
+    for _i in 10:
+        var picked := ExerciseEngine.pick_exercise_for_mission(mission, first["id"], 1)
+        assert_ne(picked["id"], first["id"])
+
+
+func test_pick_exercise_falls_back_to_any_difficulty_when_none_match() -> void:
+    var mission := ExerciseEngine.get_mission("mission_01_pre_des_mots")
+    var exercise := ExerciseEngine.pick_exercise_for_mission(mission, "", 3)
+    assert_eq(exercise["type"], "multiple_choice")