docs(known-issues): keep observed failures, drop backlog

[mrcsin]

Summary

Docs/known_issues/ had drifted from "vendor platform behaviour we cannot fix, plus the workaround" into design notes and backlog wearing the entry template. Entries 12, 14 and 15 each carried a Статус реализации: не реализовано marker; no entry in 01-11 does, because an old entry describes something already worked around in code. CLAUDE.md sends every agent to this folder before touching a failure mode, so an entry stating an unimplemented rule reads as a rule to apply.

Type of change

• docs — documentation only

Changes

Entry	Action	Why
13	deleted	No NtoLib code path changes if its rule is followed: the vendor calls CreateItems, we do not. Its one concrete point, that issued is not connected, already lives in 11 and in opc-tree-manager.md 8.3. Its incident figures contradicted 12: "1158 of 1047" against 1158 of 1228
14	deleted	No observed symptom. Its three findings are already issues #139, #140, #141. It recommended swapping SynchWihSysTree for RecreatePinMap in PlanExecutor on an untested hypothesis
15	deleted	Both halves are NtoLib defects, not vendor behaviour: the IsGroup predicate, fixed in a35fd18, and IsHistorySupport, tracked as issue #142
12	trimmed 126 to 75 lines	Keeps the incident, the symptom, why the namespace index is not ours to control, and the organizational workaround. The addressing model it restated stays in the primer, reached by a pointer
11	two links removed	They pointed at Docs/plans/iconnect-investigation-log.md, deleted in 3618ee8

Knowledge kept, not dropped: the IsHistorySupport cause moved into opc-tree-manager.md 3.4 where the consequence already was, and the IsGroup cause into the code comment in OpcProtocolAccessor.cs that needed it. Both code pointers to the deleted entry 15 are gone.

CLAUDE.md gains the gate: an entry needs an observed symptom and a workaround that exists today, even an organizational one; work not done yet is a GitHub issue.

This PR also carries the OPC UA node addressing section in masterscada-fb-primer.md that entry 12 points at, which was sitting uncommitted in the working tree.

Testing

• dotnet build NtoLib.sln — 0 errors, 0 warnings
• dotnet test NtoLib.sln — 373 passed
• terse on the two touched .cs files — exit 0
• grep for links to 13, 14, 15 and to the deleted investigation log — none left outside Docs/plans/completed/, which is an archive