Keep dropped-out signals as MISSING rows; clear the nav bar in the mark sheet

A signal absent from the latest state now stays listed (from the last
envelope seen on this gateway) and renders as missing, distinct from
null and stale as BOUNDARY.md requires; rows no longer jump.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

test/boundary_test.dart

@@ -91,4 +91,22 @@ void main() {
     await b.stop();
     await gw.server.close(force: true);
   });
+
+  test('a signal that drops out stays known as missing', () async {
+    final gw = FakeGateway();
+    await gw.start();
+    final b = Boundary(gw.url)..start();
+    await until(() => b.state == LinkState.up);
+    Map<String, dynamic> env(num v) => {'timestamp': 0, 'monotonic': 0, 'value': v, 'unit': 'x'};
+    gw.sockets.first.add(jsonEncode({'t': 'state', 'seq': 1, 'signals': {'a': env(1), 'b': env(2)}}));
+    await until(() => b.signals.length == 2);
+    gw.sockets.first.add(jsonEncode({'t': 'state', 'seq': 2, 'signals': {'a': env(3)}}));
+    await until(() => b.signals.length == 1);
+    expect(b.seen.keys, containsAll(['a', 'b']));
+    expect(b.signals.containsKey('b'), isFalse);
+    b.retarget(gw.url);
+    expect(b.seen, isEmpty);
+    await b.stop();
+    await gw.server.close(force: true);
+  });
 }