mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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/Proxy/Multiplexing/MultiplexerE2ETests.cs

@@ -172,7 +172,7 @@ public sealed class MultiplexerE2ETests
         int proxyPort = PickFreePort();
 
         // Configure three BCD addresses each width 16 for FC06 writes. The sim profile's
-        // writable HR range is [200..209] (see DL260/dl205.json's "write" list); reads
+        // writable HR range is [200..209] (see tests/sim/dl205.json's "write" list); reads
         // outside that range succeed but writes return exception 02. We use 200/202/204.
         var config = new Dictionary<string, string?>
         {

mbproxy/tests/Mbproxy.Tests/Proxy/Multiplexing/ReadCoalescingE2ETests.cs

@@ -207,7 +207,7 @@ public sealed class ReadCoalescingE2ETests
         await Task.Delay(300, TestContext.Current.CancellationToken);
 
         // Five different seeded addresses, sequential reads — none can coalesce.
-        // Selected from DL260/dl205.json's seeded ranges (200..209, 1024, 1040..1042).
+        // Selected from tests/sim/dl205.json's seeded ranges (200..209, 1024, 1040..1042).
         ushort[] addrs = [200, 201, 202, 203, 204];
         using (var client = new TcpClient())
         {