mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -148,7 +148,7 @@ public sealed class BcdTagMapBuilderTests
 
     /// <summary>
     /// Same-address entries appearing in BOTH Global AND Add are the documented
-    /// width-override pattern (design.md "Hybrid tag resolution"). They must NOT
+    /// width-override pattern (docs/Features/BcdRewriting.md "Hybrid tag resolution"). They must NOT
     /// be flagged as duplicates; Add wins.
     /// </summary>
     [Fact]