mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -170,7 +170,7 @@
     //   EvictionIntervalMs  — background eviction tick.  Scans each PLC's cache and
     //                          removes entries past their TTL.  Defaults to 5000.
     //
-    // Properties (full text in docs/design.md → "Response cache"):
+    // Properties (full text in docs/Architecture/ResponseCache.md):
     //   * Cache hits SHORT-CIRCUIT coalescing entirely (cache → coalesce → backend).
     //   * Successful FC06/FC16 write responses invalidate every cached FC03/FC04 entry
     //     whose address range OVERLAPS the write — not just exact-key match.