fix(opctreemanager): resolve an empty OPC group (#137)

[mrcsin]

Summary

FindGroup picked the target group by OpcUaScadaItem.IsGroup, which is derived (Items != null && Items.Count > 0). A folder whose children are all disabled in the OPC server GUI holds nothing, so the group was reported as not found and the rebuild refused to run. The predicate is now !IsNode || IsGroup: a folder is a container whether or not it currently holds children, and only an empty leaf pin is rejected.

Type of change

• fix — bug fix

Linked issues

Closes #137

Changes touching FB code

• XML pin IDs match the const int *PinId constants in the FB class
• New runtime-only fields carry [NonSerialized]
• [ComVisible(true)] + [Guid] untouched on existing FBs
• Read Docs/architecture/masterscada-fb-primer.md and Docs/architecture/architecture.md
• Checked Docs/known_issues/ — the root cause is written up as 15-derived-properties-as-identity.md

Tree-modifying FBs only

• Mutations are queued in UpdateData and executed from a System.Windows.Forms.Timer polling IProjectHlp.InRuntime
• Results written to the file log
• No BeginInvoke self-reposting

Testing

• dotnet build NtoLib.sln — 0 errors, 0 warnings
• dotnet test NtoLib.sln — 373 passed
• dotnet format NtoLib.sln --verify-no-changes — exit 0
• Manual host smoke: MBE group resolved at ServerInterfaces.MBE, 23 nodes constructed

Also in this PR

• a failed group lookup dumps the ScadaRootNode shape into the failure message, bounded to 3 levels, 20 children per node and 200 lines, so the message reaches the deferred pass where the Failed pin cannot
• TreeSnapshotWriter refuses to write a snapshot that holds zero nodes
• PlanBuilder refuses a plan that would restore nothing