fix(cli): resolve CLI-014..016 — re-triage update-command contract, doc-surface drift, table-column union

src/ScadaLink.CLI/Commands/CommandHelpers.cs

@@ -181,9 +181,25 @@ internal static class CommandHelpers
                     return;
                 }
 
-                var headers = items[0].ValueKind == JsonValueKind.Object
-                    ? items[0].EnumerateObject().Select(p => p.Name).ToArray()
-                    : new[] { "Value" };
+                // Derive the header set as the union of property names across *every*
+                // element, in first-seen order. Using only items[0] would silently drop
+                // columns for any later element with a different shape (CLI-016).
+                var objectItems = items.Where(i => i.ValueKind == JsonValueKind.Object).ToList();
+                string[] headers;
+                if (objectItems.Count > 0)
+                {
+                    var seen = new List<string>();
+                    var known = new HashSet<string>(StringComparer.Ordinal);
+                    foreach (var item in objectItems)
+                        foreach (var prop in item.EnumerateObject())
+                            if (known.Add(prop.Name))
+                                seen.Add(prop.Name);
+                    headers = seen.ToArray();
+                }
+                else
+                {
+                    headers = new[] { "Value" };
+                }
 
                 var rows = items.Select(item =>
                 {