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.

tests/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusProtocolOptionsTests.cs

@@ -63,7 +63,7 @@ public sealed class ModbusProtocolOptionsTests
     {
         var fake = new CapturingTransport();
         var tag = new ModbusTagDefinition("Run", ModbusRegion.Coils, 0, ModbusDataType.Bool);
-        var drv = new ModbusDriver(new ModbusDriverOptions { Host = "f", Tags = [tag] }, "m1", _ => fake);
+        var drv = new ModbusDriver(new ModbusDriverOptions { Host = "f", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } }, "m1", _ => fake);
         await drv.InitializeAsync("{}", CancellationToken.None);
 
         await drv.WriteAsync([new WriteRequest("Run", true)], CancellationToken.None);
@@ -76,7 +76,7 @@ public sealed class ModbusProtocolOptionsTests
     {
         var fake = new CapturingTransport();
         var tag = new ModbusTagDefinition("Run", ModbusRegion.Coils, 0, ModbusDataType.Bool);
-        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], UseFC15ForSingleCoilWrites = true };
+        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], UseFC15ForSingleCoilWrites = true, Probe = new ModbusProbeOptions { Enabled = false } };
         var drv = new ModbusDriver(opts, "m1", _ => fake);
         await drv.InitializeAsync("{}", CancellationToken.None);
 
@@ -90,7 +90,7 @@ public sealed class ModbusProtocolOptionsTests
     {
         var fake = new CapturingTransport();
         var tag = new ModbusTagDefinition("Sp", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16);
-        var drv = new ModbusDriver(new ModbusDriverOptions { Host = "f", Tags = [tag] }, "m1", _ => fake);
+        var drv = new ModbusDriver(new ModbusDriverOptions { Host = "f", Tags = [tag], Probe = new ModbusProbeOptions { Enabled = false } }, "m1", _ => fake);
         await drv.InitializeAsync("{}", CancellationToken.None);
 
         await drv.WriteAsync([new WriteRequest("Sp", (short)42)], CancellationToken.None);
@@ -103,7 +103,7 @@ public sealed class ModbusProtocolOptionsTests
     {
         var fake = new CapturingTransport();
         var tag = new ModbusTagDefinition("Sp", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int16);
-        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], UseFC16ForSingleRegisterWrites = true };
+        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], UseFC16ForSingleRegisterWrites = true, Probe = new ModbusProbeOptions { Enabled = false } };
         var drv = new ModbusDriver(opts, "m1", _ => fake);
         await drv.InitializeAsync("{}", CancellationToken.None);
 
@@ -118,7 +118,7 @@ public sealed class ModbusProtocolOptionsTests
         var fake = new CapturingTransport();
         // 2500 coils with cap 2000 → 2 reads (2000 + 500).
         var tag = new ModbusTagDefinition("Big", ModbusRegion.Coils, 0, ModbusDataType.Bool, ArrayCount: 2500);
-        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], MaxCoilsPerRead = 2000 };
+        var opts = new ModbusDriverOptions { Host = "f", Tags = [tag], MaxCoilsPerRead = 2000, Probe = new ModbusProbeOptions { Enabled = false } };
         var drv = new ModbusDriver(opts, "m1", _ => fake);
         await drv.InitializeAsync("{}", CancellationToken.None);