mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -420,7 +420,7 @@ internal sealed class PlcMultiplexer : IAsyncDisposable, IMultiplexCountersProvi
             {
                 // Close every attached pipe that had a request in flight; the others will
                 // simply re-issue on next request through a fresh backend connect.
-                // Per the design doc, ALL attached upstreams cascade on backend disconnect.
+                // Per docs/Architecture/ConnectionModel.md, ALL attached upstreams cascade on backend disconnect.
                 upstreamCount = _pipes.Count;
 
                 // Snapshot keys before disposal modifies the dictionary indirectly.