review(Driver.Modbus.Cli): FlushLogging() + interval validation + banner-before-events

Re-review at 7286d320. -009 FlushLogging() in finally; -010 validate --interval-ms positive
(+8 tests); -011 print subscribe banner before wiring OnDataChange (no main/poll-thread
interleave). Parity with AbCip.Cli.

src/Drivers/Cli/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Cli/Commands/ReadCommand.cs

@@ -86,6 +86,9 @@ public sealed class ReadCommand : ModbusCommandBase
         finally
         {
             await driver.ShutdownAsync(CancellationToken.None);
+            // Driver.Modbus.Cli-009: flush Serilog before process exit so buffered log lines
+            // emitted during driver shutdown are not lost. Matches Driver.AbCip.Cli pattern.
+            FlushLogging();
         }
     }