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

test/store_test.dart

@@ -77,14 +77,20 @@ void main() {
     final store = await freshStore("wipe");
     final master = randomBytes(32);
     store.setMaster(master);
+    // The store's own copy, read back before the wipe.
+    final stored = store.master()!;
     store.pinServer("example.org",
         "lm2gqd7e5q67xq3isc5hx6jfj2q7a7xvq4l7trctxipudujovmgq");
     store.markRead("cd" * 32);
 
     await store.wipe();
     // The bytes the store owned are overwritten, not just dereferenced —
-    // the caller's reference sees the zeros too.
-    expect(master.every((b) => b == 0), isTrue);
+    // the reference read back through the store sees the zeros. The
+    // caller's own buffer is the caller's to zeroize: the store holds a
+    // copy precisely so nobody else's zeroization can reach it, and vice
+    // versa a wipe never reaches a buffer the store handed out.
+    expect(stored.every((b) => b == 0), isTrue);
+    expect(master.every((b) => b == 0), isFalse);
     expect(store.master(), isNull);
     expect(store.identity(), isNull);
     expect(store.serverPin("example.org"), isNull);