mbproxy/docs: retire superseded design/plan docs and dissolve DL260/

The standalone design.md, kpi.md, operations.md, and the docs/plan/
phase tree were point-in-time planning artefacts now superseded by the
topic-organized docs/ tree (Architecture/, Features/, Operations/,
Reference/, Testing/). The DL260/ folder mixed a device-reference doc, a
test fixture, a sample test, and a screenshot; its contents now live in
their natural homes (dl205.md + mbtcp_settings.JPG under docs/Reference/,
dl205.json next to its launcher in tests/sim/, sample test dropped).

All cross-references in the surviving docs, README, CLAUDE.md, the config
template, and source comments are repointed to the new locations.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Proxy/Cache/CacheLogEvents.cs

@@ -2,7 +2,7 @@ namespace Mbproxy.Proxy.Cache;
 
 /// <summary>
 /// Source-generated <see cref="LoggerMessage"/> definitions for the response cache.
-/// Event names are stable — do not rename without updating <c>docs/design.md</c>'s
+/// Event names are stable — do not rename without updating <c>docs/Reference/LogEvents.md</c>'s
 /// Logging event-name table.
 ///
 /// <para>Levels are conservative — a busy PLC under steady cache pressure would emit one