mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -6,7 +6,7 @@ namespace Mbproxy.Proxy;
 /// BCD-rewriting PDU pipeline. Registered as the singleton <see cref="IPduPipeline"/>
 /// in production.
 ///
-/// FC scope (per design.md):
+/// FC scope (per docs/Features/BcdRewriting.md):
 ///   FC03 / FC04 response  — decode covered BCD slots from raw nibbles → binary integer.
 ///   FC06 request          — encode binary integer → BCD nibbles.
 ///   FC16 request          — per-register over the configured slots.
@@ -216,7 +216,7 @@ internal sealed class BcdPduPipeline : IPduPipeline
                 // because `9999 * 10_000 + 9999 = 99_989_999` is still <= the 32-bit
                 // BCD ceiling, so Encode32 accepts it and rewrites — losing 1 from the
                 // high word. The unconventional wire format ("two base-10000 CDAB
-                // digits", per design.md) means each word independently must be 0..9999
+                // digits", per docs/Features/BcdRewriting.md) means each word independently must be 0..9999
                 // to round-trip cleanly.
                 if (clientLow > 9999 || clientHigh > 9999)
                 {