fix(notifications): prune expired toasts and skip invisible draws

The status-bar notifications only expired the front of the queue, so an
expired newer toast could sit in front of a still-live older one and
hide it from view. The liveness check that drives continuous repaints
also disagreed with what render() actually showed.

- Expired toasts are now pruned wherever they sit in the queue, not just
  at the front, so the newest remaining toast is always a live one.
- The visible toast is the newest live entry, skipping an expired newer
  one. The liveness check reports active while a toast is visible plus
  one trailing frame, so the final faded-out state is painted once and
  then repaints stop; it self-expires if render() is skipped.

Tests: pruning removes expired entries from anywhere in the queue, the
visible toast skips an expired newest entry, and liveness tracks it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_phase4.cpp

@@ -3515,6 +3515,30 @@ void testStalenessBadge()
     }
 }
 
+void testNotificationLiveness() {
+    using namespace dragonx::ui;
+    using clk = std::chrono::steady_clock;
+    // prune removes expired entries anywhere in the queue, not just the front
+    std::deque<Notification> q;
+    q.emplace_back("long", NotificationType::Error, 30.0f);
+    q.emplace_back("short", NotificationType::Info, 1.0f);
+    q.back().created_at = clk::now() - std::chrono::seconds(2);       // expired
+    q.emplace_back("mid", NotificationType::Info, 5.0f);
+    q[2].created_at = clk::now() - std::chrono::seconds(10);          // expired
+    Notifications::pruneExpired(q, clk::now());
+    EXPECT_EQ((int)q.size(), 1);
+    EXPECT_EQ(q.back().message, std::string("long"));
+    // visibleToast skips an expired newest entry and hasActive tracks visibility
+    auto& n = Notifications::instance(); n.clear();
+    n.push("a", NotificationType::Warning, 30.0f);
+    n.push("b", NotificationType::Info, 0.001f);
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    EXPECT_TRUE(n.visibleToast() != nullptr && n.visibleToast()->message == "a");
+    EXPECT_TRUE(n.hasActive());
+    n.clear();
+    EXPECT_TRUE(!n.hasActive());
+}
+
 void testNotificationHistory()
 {
     using dragonx::ui::Notifications;
@@ -18185,6 +18209,7 @@ int main()
     testNodeStatusBanner();
     testStalenessBadge();
     testNotificationHistory();
+    testNotificationLiveness();
     testSeedMigrationResume();
     testLoggerFileSink();
     testDaemonLifecycleExecution();