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>

tests/profiles/test_save_migration.gd

@@ -0,0 +1,36 @@
+extends GutTest
+## Tests de SaveMigration : garde-fous sur la version de schema.
+
+
+func test_current_version_is_returned_unchanged() -> void:
+    var data := {"schema_version": 1, "profile_id": "profile_test"}
+    var migrated := SaveMigration.migrate(data)
+    assert_eq(migrated["schema_version"], 1)
+    assert_eq(migrated["profile_id"], "profile_test")
+
+
+func test_missing_version_is_rejected() -> void:
+    var data := {"profile_id": "profile_test"}
+    assert_eq(SaveMigration.migrate(data), {})
+
+
+func test_future_unknown_version_is_rejected_rather_than_guessed() -> void:
+    var data := {"schema_version": 99, "profile_id": "profile_test"}
+    assert_eq(SaveMigration.migrate(data), {})
+
+
+func test_zero_or_negative_version_is_rejected() -> void:
+    var data := {"schema_version": 0, "profile_id": "profile_test"}
+    assert_eq(SaveMigration.migrate(data), {})
+
+
+func test_non_numeric_version_is_rejected() -> void:
+    var data := {"schema_version": "one", "profile_id": "profile_test"}
+    assert_eq(SaveMigration.migrate(data), {})
+
+
+func test_migration_does_not_mutate_the_input_in_place() -> void:
+    var data := {"schema_version": 1, "profile_id": "profile_test"}
+    var migrated := SaveMigration.migrate(data)
+    migrated["profile_id"] = "changed"
+    assert_eq(data["profile_id"], "profile_test")