fix(focas): publish LastStatus after the fixed-tree stamp; test waits on full readiness (#673)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -1232,13 +1232,16 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
                     ProgramInfo: programInfo is not null,
                     Timers: timer is not null,
                     Status: statusProbe is not null);
-                state.LastStatus = statusProbe;
-
                 state.FixedTreeCache = new FocasFixedTreeCache(sys, [.. axes], [.. spindles], caps);
                 // The bootstrap IS an answer from the device, and Identity comes entirely from it —
                 // so stamp liveness here as well as per tick, or a CNC whose axis poll never
                 // succeeds would serve a perfectly valid sysinfo as stale.
                 state.LastFixedPollUtc = DateTime.UtcNow;
+                // Published LAST (release write): LastStatus is the field callers watch for "the first
+                // answer is in", and an authored FIXED:Status/* read is only servable once
+                // FixedTreeCache and LastFixedPollUtc are set (TryReadFixedTreeOnDevice). Publishing it
+                // first let a reader see a status while the read still returned BadCommunicationError (#673).
+                state.LastStatus = statusProbe;
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested) { return; }
             catch (Exception ex)
@@ -1998,7 +2001,12 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         /// </summary>
         public ConcurrentDictionary<string, byte> CoalesceRejected { get; } = new(StringComparer.Ordinal);
         /// <summary>Gets or sets the last <c>cnc_statinfo</c> snapshot (#257); null until the first answer.</summary>
-        public FocasStatus? LastStatus { get; set; }
+        public FocasStatus? LastStatus
+        {
+            get => Volatile.Read(ref _lastStatus);
+            set => Volatile.Write(ref _lastStatus, value);
+        }
+        private FocasStatus? _lastStatus;
         /// <summary>Gets or sets the last program information snapshot.</summary>
         public FocasProgramInfo? LastProgramInfo { get; set; }
         /// <summary>Gets or sets the cached first-axis dynamic snapshot — feeds Program/Number, /MainNumber, /Sequence.</summary>