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originally made.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/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.