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

lib/boundary.dart

@@ -39,6 +39,10 @@ class Boundary extends ChangeNotifier {
   String detail = 'not started';
   Map<String, dynamic>? hello;
   Map<String, dynamic> signals = {};
+
+  /// Last envelope of every signal this gateway has sent. A name here but not in
+  /// [signals] is missing right now — distinct from null and stale.
+  final Map<String, Map<String, dynamic>> seen = {};
   DateTime? lastState;
   DateTime? lastMessage;
 
@@ -75,6 +79,7 @@ class Boundary extends ChangeNotifier {
   /// Change the target; the current connection is dropped and the loop reconnects.
   void retarget(String newUrl) {
     url = newUrl;
+    seen.clear();
     _ws?.close();
   }
 
@@ -144,6 +149,9 @@ class Boundary extends ChangeNotifier {
         }
       case 'state':
         signals = Map<String, dynamic>.from(m['signals'] as Map);
+        signals.forEach((k, v) {
+          if (v is Map) seen[k] = Map<String, dynamic>.from(v);
+        });
         lastState = DateTime.now();
       case 'ack':
         final id = m['id'] as String?;