Migrate Workshop/PerfTest server Node Manager to AsyncCustomNodeManager

[romanett]

Part of #753.

Goal

Migrate the PerfTest Workshop server sample to AsyncCustomNodeManager. It builds a large address space by hand (no generated model), so Step 2 (Fluent) does not apply. This sample is performance-sensitive — the migration should demonstrate the benefit of the async two-tier locking model.

Scope

• Workshop/PerfTest/Server/PerfTestNodeManager.cs — PerfTestNodeManager : CustomNodeManager2
• Workshop/PerfTest/Server/PerfTest Server.csproj and its node-manager registration

Do not touch the client project.

Step 1 — AsyncCustomNodeManager

1. Base class CustomNodeManager2 → AsyncCustomNodeManager; constructor base(server, configuration, <namespaceUri>).
2. CreateAddressSpace → override async ValueTask CreateAddressSpaceAsync(...); bulk-create the large node set with await AddPredefinedNodeAsync / await AddNodeAsync.
3. Remove the global lock (Lock) around reads — rely on per-node read locking so the many simulated variables can be read in parallel. Keep writes under the base write semaphore.
4. If values are produced independently of writes (timer/simulation), consider useSamplingGroups: true on the base constructor; otherwise keep the default change-triggered manager and push updates via await ClearChangeMasksAsync.
5. Register via IAsyncNodeManagerFactory.

Acceptance criteria

• PerfTestNodeManager derives from AsyncCustomNodeManager; no CustomNodeManager2 / global lock (Lock) on the read path remains.
• PerfTest Server builds/starts; the PerfTest client runs its read/subscribe workload with results at least on par with the pre-migration baseline.

References

• docs/AsyncServerSupport.md (UA-.NETStandard) — locking model and monitored-item manager selection (useSamplingGroups).