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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusRtuFraming.cs

@@ -137,7 +137,7 @@ public static class ModbusRtuFraming
         var actual = (ushort)(frame[^2] | (frame[^1] << 8));
         if (actual != expected)
             throw new ModbusTransportDesyncException(
-                ModbusTransportDesyncException.DesyncReason.TruncatedFrame,
+                ModbusTransportDesyncException.DesyncReason.CrcMismatch,
                 $"Modbus RTU CRC mismatch: computed {expected:X4} got {actual:X4}");
 
         // Unit-address validation runs only AFTER the CRC passes: a corrupt frame is a CRC/desync
@@ -146,7 +146,7 @@ public static class ModbusRtuFraming
         var addr = frame[0];
         if (addr != expectedUnit)
             throw new ModbusTransportDesyncException(
-                ModbusTransportDesyncException.DesyncReason.TruncatedFrame,
+                ModbusTransportDesyncException.DesyncReason.UnitMismatch,
                 $"Modbus RTU unit mismatch: expected {expectedUnit} got {addr}");
 
         // Bare PDU = frame minus leading addr and trailing CRC.