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diff --git a/source/storage/Backend.mc b/source/storage/Backend.mc
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+// Author:  cirakg & claude ai
+// Summary: Key-value persistence behind an interface: Application.Storage on the watch, a dictionary in tests.
+//
+// Everything the vault persists goes through a Backend, so unit tests can run
+// against MemoryBackend (test/storage/) and simulate a full store, while the
+// app uses AppStorageBackend. Values are the types Application.Storage
+// accepts: numbers, strings, booleans, ByteArrays, arrays and dictionaries.
+// Records are single ByteArrays, so their stored size is known exactly.
+
+import Toybox.Application;
+import Toybox.Lang;
+
+module Persist {
+
+    // Base class; subclasses override all three methods.
+    class Backend {
+
+        function initialize() {
+        }
+
+        // The stored value, or null when the key is absent.
+        function getValue(key as String) as Application.Storage.ValueType {
+            return null;
+        }
+
+        // Store a value. Throws when the store is full (the vault turns that
+        // into VaultError FULL).
+        function setValue(key as String, value as Application.Storage.ValueType) as Void {
+        }
+
+        function deleteValue(key as String) as Void {
+        }
+    }
+
+    // The real store: Application.Storage, private to this app.
+    class AppStorageBackend extends Backend {
+
+        function initialize() {
+            Backend.initialize();
+        }
+
+        function getValue(key as String) as Application.Storage.ValueType {
+            return Application.Storage.getValue(key);
+        }
+
+        function setValue(key as String, value as Application.Storage.ValueType) as Void {
+            Application.Storage.setValue(key, value);
+        }
+
+        function deleteValue(key as String) as Void {
+            Application.Storage.deleteValue(key);
+        }
+    }
+}