fix: copy the master at each layer so zeroization stays local

lib/native/client.dart

@@ -67,7 +67,12 @@ class FumiNative {
   /// operation, so it never needs an isolate — and widget tests can settle
   /// it inside their fake-async zones.
   void setMaster(Uint8List master, {int? rotations}) {
-    _master = master;
+    // Own copy: the caller keeps its buffer (the onboarding screen zeroes
+    // its reference on dispose, and wipe zeroes Hive's), and the account
+    // rebuilds after register/restore/rotate must derive from untouched
+    // bytes — a shared buffer zeroized elsewhere would rebuild an account
+    // that matches none of our keys.
+    _master = Uint8List.fromList(master);
     _rebuildAccount(rotations: rotations);
   }