mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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/ProxyWorker.cs

@@ -14,7 +14,7 @@ namespace Mbproxy.Proxy;
 /// <summary>
 /// <see cref="BackgroundService"/> that owns all <see cref="PlcListenerSupervisor"/> instances.
 ///
-/// Startup posture (matches design doc "eager, continue on per-port failure"):
+/// Startup posture (matches docs/Architecture/Overview.md "eager, continue on per-port failure"):
 /// <list type="number">
 ///   <item>Enumerate <see cref="MbproxyOptions.Plcs"/> and build one supervisor per PLC.</item>
 ///   <item>Start all supervisors in parallel. Each supervisor attempts to bind immediately