refactor(modbus-rtu): distinct CrcMismatch/UnitMismatch desync reasons

The RTU framer overloaded DesyncReason.TruncatedFrame for three distinct
failures — a genuine short read, a CRC-16 validation failure, and a
CRC-valid reply from the wrong RS-485 slave. Split the latter two into
dedicated CrcMismatch / UnitMismatch enum members so a future .Reason
consumer (metrics, operator diagnostics) can tell a wiring/EMI CRC fault
apart from a mis-addressed multi-drop reply. Behaviour is unchanged — all
three still throw ModbusTransportDesyncException and tear the socket down.

Framing tests now assert the specific Reason on each path.

Follow-up #12 from the Modbus RTU-over-TCP plan.

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusRtuFramingTests.cs

@@ -43,8 +43,9 @@ public sealed class ModbusRtuFramingTests
         var frame = ModbusCrc.Append(new byte[] { 0x01, 0x03, 0x02, 0x00, 0x0A });
         frame[^1] ^= 0xFF; // corrupt CRC high byte
         await using var s = Canned(frame);
-        await Should.ThrowAsync<ModbusTransportDesyncException>(
+        var ex = await Should.ThrowAsync<ModbusTransportDesyncException>(
             ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken));
+        ex.Reason.ShouldBe(ModbusTransportDesyncException.DesyncReason.CrcMismatch);
     }
 
     [Fact]
@@ -63,8 +64,9 @@ public sealed class ModbusRtuFramingTests
         // CRC-valid FC03 frame addressed to unit 0x02, but we asked for 0x01 -> unit-mismatch desync.
         var frame = ModbusCrc.Append(new byte[] { 0x02, 0x03, 0x02, 0x00, 0x0A });
         await using var s = Canned(frame);
-        await Should.ThrowAsync<ModbusTransportDesyncException>(
+        var ex = await Should.ThrowAsync<ModbusTransportDesyncException>(
             ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken));
+        ex.Reason.ShouldBe(ModbusTransportDesyncException.DesyncReason.UnitMismatch);
     }
 
     [Fact]
@@ -86,8 +88,9 @@ public sealed class ModbusRtuFramingTests
         var frame = ModbusCrc.Append(new byte[] { 0x01, 0x03, 0x02, 0x00, 0x0A });
         var truncated = frame[..4];
         await using var s = new DribbleStream(truncated, chunk: 2);
-        await Should.ThrowAsync<ModbusTransportDesyncException>(
+        var ex = await Should.ThrowAsync<ModbusTransportDesyncException>(
             ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken));
+        ex.Reason.ShouldBe(ModbusTransportDesyncException.DesyncReason.TruncatedFrame);
     }
 
     /// <summary>