mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -317,9 +317,9 @@ curl -s http://mbproxy-host:8080/status.json |
 
 Prometheus-style scrapers should poll `/status.json` directly and translate fields into their own metric names; the service does not expose Prometheus exposition format.
 
-## Where the KPIs Live
+## Scope of This Document
 
-This document covers the **endpoint surface**: what is on the wire and how each field is computed. The **dashboard composition** — which counters roll up into which Grafana panels, alerting thresholds, fleet-aggregate definitions — lives in [`../kpi.md`](../kpi.md). Keep the two documents disjoint: when a new counter is added, list it here; when a new panel or rate calculation is added, add it to `kpi.md`.
+This document covers the **endpoint surface**: what is on the wire and how each field is computed. When a new counter is added, list it here.
 
 ## Related Documentation
 
@@ -331,4 +331,3 @@ This document covers the **endpoint surface**: what is on the wire and how each
 - [`./Configuration.md`](./Configuration.md) — `Mbproxy.AdminPort` and other option keys.
 - [`./Troubleshooting.md`](./Troubleshooting.md) — using these counters to diagnose specific failure modes.
 - [`../Reference/LogEvents.md`](../Reference/LogEvents.md) — event-id catalogue including `mbproxy.admin.bind.failed`.
-- [`../kpi.md`](../kpi.md) — dashboard catalog that consumes these counters.