chore(#550): instrument CoreScope path for joint E2E diagnosis

Info logs at each step (0xC6 reply match, hash, CoreScope GET url + result,
stored count, message-not-found) so the app debug log shows exactly where the
badge chain breaks. Part of #524.

Agent: QuietSnow (session 31eaba02)

lib/connector/meshcore_connector.dart

@@ -4742,6 +4742,10 @@ class MeshCoreConnector extends ChangeNotifier {
     }
     final key = '${parsed.timestamp}_${parsed.channelIdx}';
     final completer = _pendingPktHashCompleters[key];
+    appLogger.info(
+      '0xC6 reply hash=${parsed.hashHex} key=$key matched=${completer != null}',
+      tag: 'CoreScope',
+    );
     if (completer != null && !completer.isCompleted) {
       completer.complete(parsed);
     }
@@ -4756,8 +4760,15 @@ class MeshCoreConnector extends ChangeNotifier {
     String messageId,
   ) async {
     try {
+      appLogger.info(
+        'start ts=$ts chan=$channelIndex msg=$messageId',
+        tag: 'CoreScope',
+      );
       final pkt = await _queryPacketHash(ts, channelIndex);
-      if (pkt == null) return;
+      if (pkt == null) {
+        appLogger.info('no hash (0xC6 null/timeout)', tag: 'CoreScope');
+        return;
+      }
       _updateChannelMessageById(
         channelIndex,
         messageId,
@@ -4770,6 +4781,7 @@ class MeshCoreConnector extends ChangeNotifier {
         messageId,
         (m) => m.copyWith(coreScopeObserverCount: count),
       );
+      appLogger.info('stored observer count=$count', tag: 'CoreScope');
       notifyListeners();
     } catch (e) {
       appLogger.warn(
@@ -4787,7 +4799,14 @@ class MeshCoreConnector extends ChangeNotifier {
     final messages = _channelMessages[channelIndex];
     if (messages == null) return;
     final i = messages.indexWhere((m) => m.messageId == messageId);
-    if (i >= 0) messages[i] = transform(messages[i]);
+    if (i >= 0) {
+      messages[i] = transform(messages[i]);
+    } else {
+      appLogger.warn(
+        'message $messageId not found on chan $channelIndex; count dropped',
+        tag: 'CoreScope',
+      );
+    }
   }
 
   /// Called when a BLOCK_LIST dump ends. A truncated dump (early-END) is

lib/services/corescope_service.dart

@@ -39,6 +39,7 @@ class CoreScopeService {
         'limit': '1',
       },
     );
+    appLogger.info('GET $uri', tag: 'CoreScope');
     try {
       final resp = await _client.get(uri).timeout(timeout);
       if (resp.statusCode != 200) {
@@ -51,11 +52,19 @@ class CoreScopeService {
       final body = jsonDecode(resp.body);
       if (body is! Map) return null;
       final packets = body['packets'];
-      if (packets is! List || packets.isEmpty) return null;
+      if (packets is! List || packets.isEmpty) {
+        appLogger.info('no record yet for $packetHash', tag: 'CoreScope');
+        return null;
+      }
       final first = packets.first;
       if (first is! Map) return null;
       final count = first['observer_count'];
-      return count is num ? count.toInt() : null;
+      final result = count is num ? count.toInt() : null;
+      appLogger.info(
+        'observer_count=$result for $packetHash',
+        tag: 'CoreScope',
+      );
+      return result;
     } catch (e) {
       appLogger.warn('Query failed for $packetHash: $e', tag: 'CoreScope');
       return null;