Task #142 — Modbus multi-unit-ID per TCP connection (gateway support)

Lifts the previous "one driver = one slave" assumption so a single Modbus
driver instance can front N RTU slaves behind one Ethernet gateway (Anybus,
ProSoft, Lantronix style). Each tag carries an optional UnitId that drives
the MBAP unit-id byte per-PDU, and the IPerCallHostResolver contract surfaces
per-slave host strings so per-PLC circuit breakers fire per-slave (matches
the AB CIP template documented in docs/v2/multi-host-dispatch.md).

Changes:

- ModbusTagDefinition gains optional UnitId (byte?). Null = use driver-level
  ModbusDriverOptions.UnitId (preserves single-slave deployments verbatim).
- ResolveUnitId(tag) helper computed once per ReadOneAsync / WriteOneAsync
  call; passed through ReadRegisterBlockAsync / ReadBitBlockAsync /
  ReadRegisterBlockChunkedAsync / ReadBitBlockChunkedAsync explicitly. The
  probe loop continues using driver-level UnitId (the probe is a
  connection-health check, not slave-specific).
- ModbusDriver implements IPerCallHostResolver. ResolveHost(fullReference)
  returns "host:port/unitN" — distinct strings per slave so the resilience
  pipeline keys breakers on the right granularity. Unknown references fall
  back to the bare HostName (single-slave behaviour).
- BitInRegister RMW path also threads the per-tag UnitId through both the
  read and write halves so a multi-slave deployment stays correct under bit-
  level writes.
- Factory DTO + JSON binding extended with the per-tag UnitId field.

Tests (4 new ModbusMultiUnitTests):
- Per-tag UnitId routes to the correct slave in the MBAP header (driver-level
  UnitId=99 must NOT appear when both tags override).
- Tag without override falls back to driver-level UnitId.
- IPerCallHostResolver returns distinct "host:port/unitN" strings per slave.
- Unknown reference returns the bare HostName fallback.

Existing 220 unit tests + 107 addressing tests still green. Per-PLC breaker
isolation under simulated dead slaves is verifiable via the existing AB CIP
test infra; live coverage lands as an integration test in the #138 docs/e2e
refresh.

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

@@ -116,7 +116,8 @@ public static class ModbusDriverFactoryExtensions
                     : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, name, driverInstanceId, "StringByteOrder"),
                 WriteIdempotent: t.WriteIdempotent ?? false,
                 ArrayCount: parsed.ArrayCount,
-                Deadband: t.Deadband);
+                Deadband: t.Deadband,
+                UnitId: t.UnitId);
         }
 
         return new ModbusTagDefinition(
@@ -136,7 +137,8 @@ public static class ModbusDriverFactoryExtensions
                 : ParseEnum<ModbusStringByteOrder>(t.StringByteOrder, t.Name, driverInstanceId, "StringByteOrder"),
             WriteIdempotent: t.WriteIdempotent ?? false,
             ArrayCount: t.ArrayCount,
-            Deadband: t.Deadband);
+            Deadband: t.Deadband,
+            UnitId: t.UnitId);
     }
 
     private static T ParseEnum<T>(string? raw, string? tagName, string driverInstanceId, string field) where T : struct, Enum
@@ -221,6 +223,7 @@ public static class ModbusDriverFactoryExtensions
         public bool? WriteIdempotent { get; init; }
         public int? ArrayCount { get; init; }
         public double? Deadband { get; init; }
+        public byte? UnitId { get; init; }
     }
 
     internal sealed class ModbusProbeDto