fix(hosts): a driver that never built a tag table reports NO ANSWER, not zero skips

Found on the docker-dev live gate, not in tests. Two drivers that faulted
during initialise (rtu-gate, opcua1) each rendered "0 tags" on /hosts against
two authored tags apiece, because a ledger that had never committed returned
SkippedTagReport.Empty.

A zero report is a positive claim -- "this driver skipped nothing" -- and
making it on behalf of a driver that never got as far as looking is exactly
the quiet inaccuracy #541 exists to remove. GetSkippedTags() and
SkippedTagLedger.Snapshot() now return null until the first committed build,
so the column renders the same em-dash it already used for a driver that
cannot report at all. Both are 'no answer'; neither is 'nothing skipped'.

Knock-on: a FOCAS driver whose only init threw now reports null rather than a
zero report, which is strictly more honest than the assertion the test
originally made.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISkippedTagReporter.cs

@@ -44,8 +44,15 @@ public interface ISkippedTagReporter
     ///     Must be a pure in-memory read with no I/O — it is called on the health-publish path, is
     ///     deliberately NOT routed through the resilience pipeline, and must never block.
     /// </remarks>
-    /// <returns>The current report; never null. An all-zero report means "nothing skipped".</returns>
-    SkippedTagReport GetSkippedTags();
+    /// <returns>
+    ///     The report from the driver's most recent tag-table build, or <see langword="null"/> when it has
+    ///     not built one yet — a driver that faulted during initialise has <b>no answer</b>, which is not
+    ///     the same as the answer "nothing skipped". Returning a zero report there would state that a
+    ///     driver with a dozen authored tags has none, which is the class of quiet inaccuracy this whole
+    ///     capability exists to remove. (Found on the docker-dev rig, where two faulted drivers each
+    ///     rendered "0 tags" against two authored ones.)
+    /// </returns>
+    SkippedTagReport? GetSkippedTags();
 }
 
 /// <summary>One authored tag a driver could not build into its poll table, and why.</summary>
@@ -71,9 +78,6 @@ public sealed record SkippedTag(string RawPath, string Reason);
 /// </param>
 public sealed record SkippedTagReport(int AuthoredCount, int SkippedCount, IReadOnlyList<SkippedTag> Sample)
 {
-    /// <summary>A driver that has authored nothing and skipped nothing.</summary>
-    public static readonly SkippedTagReport Empty = new(0, 0, []);
-
     /// <summary>Whether any authored tag was skipped — i.e. whether this is worth an operator's attention.</summary>
     public bool HasSkips => SkippedCount > 0;
 
@@ -113,7 +117,9 @@ public sealed class SkippedTagLedger
     public const int MaxSample = 20;
 
     private readonly object _gate = new();
-    private SkippedTagReport _visible = SkippedTagReport.Empty;
+
+    /// <summary>Null until the first <see cref="Commit"/> — "no answer yet", never a zero report.</summary>
+    private SkippedTagReport? _visible;
 
     private int _stagedAuthored;
     private int _stagedSkipped;
@@ -168,8 +174,9 @@ public sealed class SkippedTagLedger
     }
 
     /// <summary>The current published snapshot.</summary>
-    /// <returns>The last committed report, or <see cref="SkippedTagReport.Empty"/> before the first commit.</returns>
-    public SkippedTagReport Snapshot()
+    /// <returns>The last committed report, or <see langword="null"/> before the first commit — a driver
+    /// that has not built a tag table has no answer, and a zero report would be a false one.</returns>
+    public SkippedTagReport? Snapshot()
     {
         lock (_gate) return _visible;
     }

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs

@@ -31,7 +31,7 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     private AbCipDriverOptions _options;
     private readonly string _driverInstanceId;

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -17,7 +17,7 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     /// <summary>Mutable so <see cref="InitializeAsync"/> can adopt a re-parsed config on reinitialize
     /// (#516). Only ever written on the init path, before any reader/session uses it.</summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Calculation/CalculationDriver.cs

@@ -42,7 +42,7 @@ public sealed class CalculationDriver : IDriver, IDependencyConsumer, ISubscriba
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     // OPC UA status codes surfaced by the calc driver.
     private const uint StatusGood = 0u;

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

@@ -27,7 +27,7 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     /// <summary>Mutable so <see cref="InitializeAsync"/> can adopt a re-parsed config (#516). Written only
     /// on the init path, together with <see cref="_clientFactory"/>, before either is used.</summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -45,7 +45,7 @@ public sealed class GalaxyDriver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     private readonly string _driverInstanceId;
     private readonly GalaxyDriverOptions _options;

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs

@@ -79,7 +79,7 @@ public sealed class MTConnectDriver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     /// <summary>
     ///     The Agent could not be reached, answered unusably, or blew the per-call deadline. Distinct

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -25,7 +25,7 @@ public sealed class ModbusDriver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     // ---- instance fields (grouped at top for auditability) ----
 

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttDriver.cs

@@ -70,7 +70,7 @@ public sealed class MqttDriver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     // ---- Write status codes (Gitea #508) ----
     //

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -37,7 +37,7 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     /// <summary>
     ///     Recomputes the skip report from the freshly-built map. Derived by re-asking the map to resolve

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -36,7 +36,7 @@ public sealed class S7Driver
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     private readonly string _driverInstanceId;
     private readonly ILogger<S7Driver> _logger;

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Sql/SqlDriver.cs

@@ -46,7 +46,7 @@ public sealed class SqlDriver
     private IReadOnlyList<SkippedTag> _blobSkips = [];
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     /// <summary>
     ///     Republishes the skip ledger as the union of the blob-level skips and the catalog gate's

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -17,7 +17,7 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
     private readonly SkippedTagLedger _skippedTags = new();
 
     /// <inheritdoc />
-    public SkippedTagReport GetSkippedTags() => _skippedTags.Snapshot();
+    public SkippedTagReport? GetSkippedTags() => _skippedTags.Snapshot();
 
     // Mutable so ReinitializeAsync can apply a new config generation. The constructor seeds
     // it; InitializeAsync re-parses driverConfigJson over the top of it.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Hosts.razor

@@ -234,8 +234,11 @@ else
                                                Healthy, Errors/5 min stays 0, Last error stays empty, and Last
                                                read keeps advancing off the tags that DID map. On the Makino in
                                                #540 that read as a perfectly healthy driver ignoring 20 of its 22
-                                               tags. "—" means the driver cannot report skips, which is distinct
-                                               from reporting none. *@
+                                               tags. "—" means NO ANSWER — either the driver cannot report skips,
+                                               or it faulted before building a tag table — which is distinct from
+                                               the positive claim "this driver skipped nothing". A pre-fix build
+                                               rendered "0 tags" for two faulted drivers on the docker-dev rig
+                                               that had two authored tags apiece. *@
                                             @if (d.SkippedTags is null)
                                             {
                                                 <span class="text-muted">—</span>

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Hosts/HostsDriverView.cs

@@ -44,9 +44,10 @@ public sealed record HostsDriverInstanceInfo(string DriverInstanceId, string Clu
 /// <param name="HostStatuses">Per-host connectivity for a multi-device driver, ordered by host name; null
 ///     when the driver reports no per-host detail. Lets one unreachable device show even while the driver
 ///     row itself is Healthy.</param>
-/// <param name="SkippedTags">Authored tags the driver could not build into its poll table; null when the
-///     driver cannot report them. Lets "ignoring 20 of 22 tags" show even while every other column on the
-///     row — state, last read, error count, last error — reads perfectly healthy (Gitea #541).</param>
+/// <param name="SkippedTags">Authored tags the driver could not build into its poll table; null when there
+///     is no answer — the driver cannot report skips, or it faulted before building a tag table. Lets
+///     "ignoring 20 of 22 tags" show even while every other column on the row — state, last read, error
+///     count, last error — reads perfectly healthy (Gitea #541).</param>
 public sealed record HostsDriverRow(
     string DriverInstanceId, string? Name, string? DriverType, string State,
     DateTime? LastSuccessfulReadUtc, string? LastError, int ErrorCount5Min, DateTime PublishedUtc,