fix: read back sent copies after the sec 5.6 upstream fix, pinned at 2d65d66

test/ffi_smoke_test.dart

@@ -0,0 +1,19 @@
+// Spike: the first Dart-to-Rust link. Opens the built cdylib and calls the
+// smoke entry point, which round-trips an address through fumi-core's
+// in-memory store. Proves the dynamic-library bridge works; the real binding
+// replaces this once the surface settles.
+
+import "dart:ffi";
+import "dart:io";
+
+import "package:flutter_test/flutter_test.dart";
+
+void main() {
+  test("native library links and round-trips", () {
+    final lib = DynamicLibrary.open(
+        "${Directory.current.path}/native/target/release/libsmol_mail_native.so");
+    final smolSmoke =
+        lib.lookupFunction<Int32 Function(), int Function()>("smol_smoke");
+    expect(smolSmoke(), 0);
+  });
+}