PubSub: implement High Availability / Redundancy (Part 14)

[marcschier]

Tracks implementing High Availability / Redundancy for OPC UA PubSub (Part 14 §9.1.6 / transparent & non-transparent redundancy).

The 2.0 PubSub stack already externalizes the state needed for HA deployments:

• IPubSubIdAllocator (Libraries/Opc.Ua.PubSub/Configuration/IPubSubIdAllocator.cs) for shared, monotonically-increasing id allocation across instances.
• Pluggable configuration/runtime-state stores and HA runtime-state providers with failover.

Remaining work to make PubSub redundancy first-class:

• Implement the PubSub redundancy modes (transparent / non-transparent / hot / warm / cold) per Part 14.
• A distributed/HA implementation of IPubSubIdAllocator (e.g. backed by the shared state store) with documented monotonicity guarantees.
• Group-level active/standby coordination and failover for writer/reader groups.
• Documentation and integration tests for multi-instance failover.

Raised from PR #3892 review feedback (comment on IPubSubIdAllocator.cs).