Add profile and save persistence, wired into the three autoloads

scripts/profiles/: SaveIO handles real file I/O in user://saves (one
file per profile_id, atomic write via temp-file-then-rename, backup
copy before each overwrite, corrupted-main falls back to backup,
schema-invalid content is rejected like corruption). SaveMigration
and SaveFactory are pure; SaveMigration refuses a schema_version it
doesn't recognize (older or newer) rather than guessing. ProgressUpdates
is pure and wires ReviewScheduler/DifficultyManager onto a save's
skills dict, including per-session overdue-review marking.

The three autoloads (SaveManager, ProfileManager, ProgressManager)
stay thin: in-memory current-profile state and orchestration only,
no logic of their own, so the actual behavior lives in the tested
pure/file-IO modules above rather than in untestable glue.

26 new headless tests, including the failure modes called out in the
launch prompt: a truncated .tmp file never corrupts the readable
save, a corrupted main file falls back to its backup, two profiles
never mix (down through the real autoload layer, not just SaveIO),
and an unknown future schema_version is rejected outright.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

autoload/profile_manager.gd

@@ -1,3 +1,35 @@
 extends Node
-## Creation, selection et suppression des profils enfants.
-## Logique detaillee a partir de l'etape "Profils et sauvegardes".
+## Creation, selection et suppression des profils enfants. Chaque profil a un
+## profile_id distinct ; SaveManager garantit qu'un profil ne lit ni n'ecrit
+## jamais le fichier d'un autre (voir "Points durs" du prompt de lancement).
+
+var current_profile_id: String = ""
+
+
+func create_profile(display_name: String, school_level: String) -> String:
+    var profile_id := _generate_profile_id()
+    SaveManager.load_or_create(profile_id, display_name, school_level)
+    return profile_id
+
+
+## Charge le profil dans ProgressManager et le marque comme profil courant.
+## Ne fait rien si le profil n'existe pas.
+func select_profile(profile_id: String) -> bool:
+    if not ProgressManager.load_profile(profile_id):
+        return false
+    current_profile_id = profile_id
+    return true
+
+
+func delete_profile(profile_id: String) -> void:
+    SaveManager.delete(profile_id)
+    if current_profile_id == profile_id:
+        current_profile_id = ""
+
+
+func list_profiles() -> Array[String]:
+    return SaveManager.list_profile_ids()
+
+
+func _generate_profile_id() -> String:
+    return "profile_%d_%d" % [Time.get_unix_time_from_system(), randi() % 10000]