feat(mesh-phase4): DriverHostActor runs ConfigDb-free (LocalDb alarm store, guarded acks)

Nullable ConfigDb factory; UpsertNodeDeploymentState no-ops when absent
(central persists acks from the ApplyAck); scripted-alarm condition state
served from the LocalDb store instead of the ConfigDb-backed Ef store.
Combines Phase-4 Tasks 4 + 6. Removes the interim dbFactory! cast.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/ScriptedAlarms/LocalDbAlarmConditionStateStoreTests.cs

@@ -121,6 +121,45 @@ public sealed class LocalDbAlarmConditionStateStoreTests : IDisposable
         loaded.LastConfirmUtc.ShouldBe(state.LastConfirmUtc);
         loaded.LastConfirmUser.ShouldBe("bob");
         loaded.LastConfirmComment.ShouldBe("confirm-comment");
+        // Shouldly's DateTime ShouldBe compares ticks and ignores DateTimeKind, so an explicit Kind
+        // assertion is the only guard on the Utc-kind contract the round-trip ("O") format encodes.
+        loaded.LastTransitionUtc.Kind.ShouldBe(DateTimeKind.Utc);
+    }
+
+    /// <summary>Unset audit fields (LastAckUtc/User, LastConfirmUtc/User) round-trip back as null.</summary>
+    [Fact]
+    public async Task Null_audit_fields_round_trip_as_null()
+    {
+        var store = NewStore();
+        var t = new DateTime(2026, 06, 10, 12, 00, 00, DateTimeKind.Utc);
+        var state = new AlarmConditionState(
+            AlarmId: "alarm-null-audit",
+            Enabled: AlarmEnabledState.Enabled,
+            Active: AlarmActiveState.Inactive,
+            Acked: AlarmAckedState.Unacknowledged,
+            Confirmed: AlarmConfirmedState.Unconfirmed,
+            Shelving: ShelvingState.Unshelved,
+            LastTransitionUtc: t,
+            LastActiveUtc: null,
+            LastClearedUtc: null,
+            LastAckUtc: null,
+            LastAckUser: null,
+            LastAckComment: null,
+            LastConfirmUtc: null,
+            LastConfirmUser: null,
+            LastConfirmComment: null,
+            Comments: ImmutableList<AlarmComment>.Empty);
+
+        await store.SaveAsync(state, CancellationToken.None);
+        var loaded = await store.LoadAsync("alarm-null-audit", CancellationToken.None);
+
+        loaded.ShouldNotBeNull();
+        loaded.LastAckUtc.ShouldBeNull();
+        loaded.LastAckUser.ShouldBeNull();
+        loaded.LastAckComment.ShouldBeNull();
+        loaded.LastConfirmUtc.ShouldBeNull();
+        loaded.LastConfirmUser.ShouldBeNull();
+        loaded.LastConfirmComment.ShouldBeNull();
     }
 
     /// <summary>Loading an id that was never saved returns null.</summary>