opcua: raise a comms-lost fault when the OPC UA connection drops

[mfaferek93]

Problem

When the OPC UA plugin loses its connection to the server (PLC powered off, cable/network fault, session drop), the poller sets snapshot_.connected = false and enters its reconnect-with-backoff loop (opcua_poller.cpp reconnect arm, ~L849), and the connected flag is surfaced on the x-plc-status view (opcua_plugin.cpp, j["connected"]). But no fault is raised on /faults.

So a controller going dark is invisible to /faults consumers: an operator watching the fault list sees nothing, even though the plugin has just gone blind to everything that PLC was reporting. The only signal is the connected boolean on a status endpoint that fault consumers do not watch. ISA-18.2 treats loss of communication with a controller as an alarm-worthy condition.

Proposal

Raise a single, component-scoped comms-lost fault:

• Fault code PLC_COMMS_LOST, severity ERROR (configurable).
• Raised on the PLC component (the plugin's root PLC component/source), not per data point.
• Debounced: only raise once the connection has been continuously down for a configurable window (default ~5s), so a brief blip during a normal reconnect does not flap a fault. Idempotent (raise once, not every poll cycle).
• Cleared on successful reconnect.
• Config knobs: comms_lost_fault_enabled (default true), comms_lost_debounce_ms (default 5000).

This is a connection-state transition, so it belongs in the poller's reconnect arm using the existing report_fault / clear_fault path, not the value-evaluation path.

Notes

• report_fault / clear_fault clients already exist (opcua_plugin.cpp), and the PLC component the fault should attach to already exists (opcua_plugin.cpp, "PLC runtime connected at ...").
• Keeps /faults semantics uniform across PLC sources.