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/save_manager.gd

@@ -1,4 +1,39 @@
 extends Node
 ## Serialisation des sauvegardes par profil dans user:// (ecriture atomique,
-## copie de secours, migration).
-## Logique detaillee a partir de l'etape "Profils et sauvegardes".
+## copie de secours, migration). Delegue le travail reel a SaveIO,
+## SaveFactory et SaveMigration (scripts/profiles/), qui restent testables
+## sans arbre de scenes.
+
+var _save_schema: Dictionary = {}
+
+
+func _ready() -> void:
+    _save_schema = DataLoader.load_schema("save")
+
+
+## Cree une nouvelle sauvegarde si le profil n'en a pas encore.
+func load_or_create(profile_id: String, display_name: String, school_level: String) -> Dictionary:
+    var existing: Variant = SaveIO.read(profile_id, _save_schema)
+    if existing != null:
+        return existing
+    var created := SaveFactory.new_save(profile_id, display_name, school_level)
+    SaveIO.write(profile_id, created)
+    return created
+
+
+## Lit la sauvegarde d'un profil existant. Null si elle est introuvable (et sa
+## copie de secours aussi).
+func load(profile_id: String) -> Variant:
+    return SaveIO.read(profile_id, _save_schema)
+
+
+func save(profile_id: String, data: Dictionary) -> bool:
+    return SaveIO.write(profile_id, data)
+
+
+func delete(profile_id: String) -> void:
+    SaveIO.delete(profile_id)
+
+
+func list_profile_ids() -> Array[String]:
+    return SaveIO.list_profile_ids()