mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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/sim/run-dl205-sim.ps1

@@ -9,7 +9,7 @@
     port; the server process stays attached so Ctrl-C (or parent exit) kills it cleanly.
 
     pymodbus version pin: 3.13.0
-    (Matches the profile comment in DL260/dl205.json. Record the version here AND in
+    (Matches the profile comment in dl205.json. Record the version here AND in
     tests/sim/README.md so it is never lost across re-provisioning.)
 
     API note: pymodbus 3.13.0 uses 'pymodbus.simulator' (not the legacy 'pymodbus.server
@@ -19,8 +19,8 @@
     pymodbus.
 
 .PARAMETER Profile
-    Path to the pymodbus JSON profile. Defaults to ../../DL260/dl205.json relative to
-    this script's directory (i.e. the checked-in DL205 quirk profile).
+    Path to the pymodbus JSON profile. Defaults to dl205.json in this script's
+    directory (i.e. the checked-in DL205 quirk profile).
 
 .PARAMETER Port
     TCP port for the Modbus server to listen on. Defaults to 5020.
@@ -33,7 +33,7 @@
 #>
 [CmdletBinding()]
 param(
-    [string]$Profile = (Join-Path $PSScriptRoot '..\..\DL260\dl205.json'),
+    [string]$Profile = (Join-Path $PSScriptRoot 'dl205.json'),
     [int]$Port = 5020
 )