feat(storage): encrypted wallet file and persistence

src/Core/Storage/WalletStore.cs

@@ -0,0 +1,38 @@
+namespace PalladiumWallet.Core.Storage;
+
+/// <summary>
+/// Lettura/scrittura del file wallet su disco (blueprint §8): JSON in chiaro
+/// senza password, contenitore AES-GCM con password. Scrittura atomica
+/// (file temporaneo + rename) per non corrompere il wallet su crash.
+/// </summary>
+public static class WalletStore
+{
+    public static bool Exists(string path) => File.Exists(path);
+
+    /// <summary>True se il file richiede una password per l'apertura.</summary>
+    public static bool RequiresPassword(string path) =>
+        EncryptedFile.IsEncrypted(File.ReadAllText(path));
+
+    public static WalletDocument Load(string path, string? password = null)
+    {
+        var content = File.ReadAllText(path);
+        if (EncryptedFile.IsEncrypted(content))
+        {
+            if (string.IsNullOrEmpty(password))
+                throw new WrongPasswordException();
+            content = EncryptedFile.Decrypt(content, password);
+        }
+        return WalletDocument.FromJson(content);
+    }
+
+    public static void Save(WalletDocument doc, string path, string? password = null)
+    {
+        var content = doc.ToJson();
+        if (!string.IsNullOrEmpty(password))
+            content = EncryptedFile.Encrypt(content, password);
+
+        var tmp = path + ".tmp";
+        File.WriteAllText(tmp, content);
+        File.Move(tmp, path, overwrite: true);
+    }
+}