Task #141 — Modbus subscribe-side knobs (deadband + write-on-change)

Two driver-side filters that ≥5 of 6 surveyed vendors expose:

1. Per-tag Deadband (double?, on ModbusTagDefinition) — when set, the
   PollGroupEngine onChange callback suppresses publishes whose distance
   from the last-published value is below the threshold. Reduces wire
   traffic to OPC UA clients on noisy analog signals (flow meters,
   temperatures). Numeric scalar types only — Bool / BitInRegister / String
   / array tags publish unconditionally.

2. WriteOnChangeOnly (bool, on ModbusDriverOptions) — when true, the driver
   short-circuits writes whose value matches the most recent successful
   write to that tag. Saves PLC bandwidth on clients that re-publish the
   same setpoint every scan. Cache invalidates on any read that returns a
   different value, so HMI-side changes don't get masked.

Both default off so existing deployments see no behaviour change.

Implementation:
- ShouldPublish guard wraps the existing OnDataChange invocation. First sample
  always passes through (no baseline); subsequent samples compare via
  Convert.ToDouble for the cross-numeric-type math.
- IsRedundantWrite check at the top of WriteAsync; on success the cache is
  populated. Object.Equals handles boxed-numeric equality; arrays are
  excluded (reference-equality would never match anyway).
- ReadAsync invalidates the WriteOnChangeOnly cache when the new value
  differs from the cached last-written value.

Tests (5 new ModbusSubscribeOptionsTests):
- Deadband suppresses sub-threshold changes (100 → 102 → 106 → 107 with
  deadband=5 publishes 100 and 106 only).
- Deadband=null still publishes every change.
- WriteOnChangeOnly suppresses 3 identical 42 writes (only first hits wire).
- WriteOnChangeOnly default false hits the wire every time.
- Read-divergence cache invalidation: external panel write to 99, our
  client's re-write of 42 must NOT be suppressed.

220/220 unit tests green; existing ProtocolOptions tests hardened against
probe-loop noise by disabling the probe in their fixtures.

src/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverFactoryExtensions.cs

@@ -45,6 +45,7 @@ public static class ModbusDriverFactoryExtensions
             UseFC15ForSingleCoilWrites = dto.UseFC15ForSingleCoilWrites ?? false,
             UseFC16ForSingleRegisterWrites = dto.UseFC16ForSingleRegisterWrites ?? false,
             DisableFC23 = dto.DisableFC23 ?? false,
+            WriteOnChangeOnly = dto.WriteOnChangeOnly ?? false,
             AutoReconnect = dto.AutoReconnect ?? true,
             Tags = dto.Tags is { Count: > 0 }
                 ? [.. dto.Tags.Select(t => BuildTag(t, driverInstanceId))]
@@ -102,7 +103,8 @@ public static class ModbusDriverFactoryExtensions
                     ? ModbusStringByteOrder.HighByteFirst
                     : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, name, driverInstanceId, "StringByteOrder"),
                 WriteIdempotent: t.WriteIdempotent ?? false,
-                ArrayCount: parsed.ArrayCount);
+                ArrayCount: parsed.ArrayCount,
+                Deadband: t.Deadband);
         }
 
         return new ModbusTagDefinition(
@@ -121,7 +123,8 @@ public static class ModbusDriverFactoryExtensions
                 ? ModbusStringByteOrder.HighByteFirst
                 : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, t.Name, driverInstanceId, "StringByteOrder"),
             WriteIdempotent: t.WriteIdempotent ?? false,
-            ArrayCount: t.ArrayCount);
+            ArrayCount: t.ArrayCount,
+            Deadband: t.Deadband);
     }
 
     private static T ParseEnum<T>(string? raw, string? tagName, string driverInstanceId, string field) where T : struct, Enum
@@ -155,6 +158,7 @@ public static class ModbusDriverFactoryExtensions
         public bool? UseFC15ForSingleCoilWrites { get; init; }
         public bool? UseFC16ForSingleRegisterWrites { get; init; }
         public bool? DisableFC23 { get; init; }
+        public bool? WriteOnChangeOnly { get; init; }
         public bool? AutoReconnect { get; init; }
         public List<ModbusTagDto>? Tags { get; init; }
         public ModbusProbeDto? Probe { get; init; }
@@ -202,6 +206,7 @@ public static class ModbusDriverFactoryExtensions
         public string? StringByteOrder { get; init; }
         public bool? WriteIdempotent { get; init; }
         public int? ArrayCount { get; init; }
+        public double? Deadband { get; init; }
     }
 
     internal sealed class ModbusProbeDto