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>

scripts/profiles/save_migration.gd

@@ -0,0 +1,30 @@
+class_name SaveMigration
+extends RefCounted
+## Migration des sauvegardes entre versions de schema. Code pur. Une seule
+## version existe pour l'instant (1) ; chaque montee de version future ajoute
+## une etape a la chaine ci-dessous, sans jamais reecrire les etapes passees.
+
+const CURRENT_SCHEMA_VERSION := 1
+
+
+## Retourne un dictionnaire vide si la version est absente, invalide, ou plus
+## recente que ce que cette version du moteur sait lire : refuser plutot que
+## de deviner et corrompre la progression de l'enfant.
+static func migrate(data: Dictionary) -> Dictionary:
+    if not data.has("schema_version"):
+        return {}
+    var version: Variant = data["schema_version"]
+    if typeof(version) != TYPE_INT and typeof(version) != TYPE_FLOAT:
+        return {}
+    var version_int: int = int(version)
+    if version_int < 1 or version_int > CURRENT_SCHEMA_VERSION:
+        return {}
+
+    var migrated: Dictionary = data.duplicate(true)
+    # Chaine de montees de version, a completer quand CURRENT_SCHEMA_VERSION
+    # augmentera :
+    # if version_int == 1:
+    #     migrated = _migrate_v1_to_v2(migrated)
+    #     version_int = 2
+    migrated["schema_version"] = CURRENT_SCHEMA_VERSION
+    return migrated