test(e2e): assert vault isolation actually isolated

After each e2e App::load(), assert the loaded vault is empty. An
isolated run that loads any profile can only mean load_vault() fell
through to legacy migration (real repo vault with user keys). The bug
fixed in the previous commit passed green for weeks precisely because
nothing checked this; the guard makes it fail loudly and immediately.

tests/nip46_e2e.rs

@@ -354,7 +354,16 @@ async fn nip46_client_handshake_and_sign_against_fake_amber() {
         )
         .unwrap();
         std::env::set_var("XDG_DATA_HOME", &tmp);
-        std::sync::Arc::new(Mutex::new(App::load().expect("load app")))
+        let app = std::sync::Arc::new(Mutex::new(App::load().expect("load app")));
+        // Guard: if seeding ever misses the real data_dir path again,
+        // load_vault() silently migrates the legacy repo vault (real user
+        // keys) into the test. An isolated run must load an EMPTY vault.
+        assert!(
+            app.try_lock().unwrap().vault.profiles.is_empty(),
+            "e2e vault isolation failed: a non-empty vault was loaded — \
+             the seeded vault is not where data_dir() looks"
+        );
+        app
     };
 
     // Amber's keys: `comms` is the per-connection key in the bunker:// URI;
@@ -650,7 +659,14 @@ async fn run_qr_pairing(connect_shape: &'static str) {
         )
         .unwrap();
         std::env::set_var("XDG_DATA_HOME", &tmp);
-        std::sync::Arc::new(Mutex::new(App::load().expect("load app")))
+        let app = std::sync::Arc::new(Mutex::new(App::load().expect("load app")));
+        // Same empty-vault guard as the sibling test above.
+        assert!(
+            app.try_lock().unwrap().vault.profiles.is_empty(),
+            "e2e vault isolation failed: a non-empty vault was loaded — \
+             the seeded vault is not where data_dir() looks"
+        );
+        app
     };
     {
         let mut a = app.lock().await;