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>

autoload/profile_manager.gd

@@ -31,5 +31,22 @@ func list_profiles() -> Array[String]:
     return SaveManager.list_profile_ids()
 
 
+## Resume de chaque profil pour l'ecran de selection : profile_id, display_name
+## et school_level, sans charger toute la progression.
+func list_profile_summaries() -> Array[Dictionary]:
+    var summaries: Array[Dictionary] = []
+    for profile_id in list_profiles():
+        var data: Variant = SaveManager.load(profile_id)
+        if data == null:
+            continue
+        var summary := {
+            "profile_id": profile_id,
+            "display_name": data["display_name"],
+            "school_level": data["school_level"],
+        }
+        summaries.append(summary)
+    return summaries
+
+
 func _generate_profile_id() -> String:
     return "profile_%d_%d" % [Time.get_unix_time_from_system(), randi() % 10000]