Move ModbusDriverOptions (and companion option types) to a new
Driver.Modbus.Contracts sibling project. The contracts project
references only Driver.Modbus.Addressing (itself zero-dep and
Admin-safe) because ModbusDriverOptions.Probe/Family/Region
properties use enum types that live there.
Drop 'using ZB.MOM.WW.OtOpcUa.Core.Abstractions' and replace
<see cref="IHostConnectivityProbe"/> with <c>IHostConnectivityProbe</c>
per the approved decision — the using was doc-comment-only.
The runtime Driver.Modbus project gains a ProjectReference back to
.Contracts; the .slnx is updated accordingly.