Detailed Description

Southbound protocol-driver framework (DETWS_ENABLE_SOUTHBOUND).

The northbound surface of this library is HTTP/WS/SNMP/etc. to a controller; the southbound surface is the field devices it polls and drives (a Modbus slave, a BACnet controller, a raw sensor over SPI/I2C/UART). Today Modbus master is the one such driver, hand-wired by the app. This is the uniform seam every southbound driver plugs into, so the app addresses any field device the same way regardless of the wire protocol underneath: register a driver (a small vtable + its instance context), then read/write points (registers, coils, objects) by driver name through one facade.

One owner, one API: the framework owns driver lookup + dispatch; each driver owns its own transport (it is handed the point id and returns the value, doing whatever Modbus/BACnet/GPIO I/O it needs). The block (matrix) read/write is the atomic multi-point path - a contiguous span of points moved in one driver call, which a protocol like Modbus can satisfy as a single request.

Pure registry + dispatch: no heap, no stdlib, host-testable with a fake driver. Drivers are borrowed (the caller keeps the SouthboundDriver + its ctx alive for the registry's lifetime).

Definition in file southbound.h.