mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

@@ -4,7 +4,7 @@ The proxy holds one persistent backend TCP socket per PLC and multiplexes many u
 
 ## Why One Backend Connection Per PLC
 
-An earlier design opened a fresh backend socket for each accepted upstream client (1:1 pairs). That model collapsed against the **AutomationDirect H2-ECOM100**, which caps simultaneous TCP clients at **4 per PLC** (see [`../../DL260/dl205.md`](../../DL260/dl205.md) under "Behavioural Oddities"). The fifth upstream client to attach to a busy PLC was refused at connect, with no recourse other than waiting for an existing pair to drop.
+An earlier design opened a fresh backend socket for each accepted upstream client (1:1 pairs). That model collapsed against the **AutomationDirect H2-ECOM100**, which caps simultaneous TCP clients at **4 per PLC** (see [`../Reference/dl205.md`](../Reference/dl205.md) under "Behavioural Oddities"). The fifth upstream client to attach to a busy PLC was refused at connect, with no recourse other than waiting for an existing pair to drop.
 
 Multiplexing replaces 1:N upstream-to-backend with N:1 upstream-to-multiplexer-to-backend:
 
@@ -244,4 +244,4 @@ The per-request timeout watchdog described above is the production defence again
 - [`../Operations/StatusPage.md`](../Operations/StatusPage.md) — `inFlight`, `maxInFlight`, `txIdWraps`, `queueDepth`, `disconnectCascades` counters
 - [`../Reference/LogEvents.md`](../Reference/LogEvents.md) — `mbproxy.multiplex.*` structured log events
 - [`../Testing/Simulator.md`](../Testing/Simulator.md) — pymodbus 3.13.0 deferred-handler quirk in detail
-- [`../../DL260/dl205.md`](../../DL260/dl205.md) — DL205/DL260 quirks including the 4-client ECOM cap
+- [`../Reference/dl205.md`](../Reference/dl205.md) — DL205/DL260 quirks including the 4-client ECOM cap

mbproxy/docs/Architecture/Overview.md

@@ -145,6 +145,4 @@ The simulator used by the end-to-end test suite — a `pymodbus`-based stand-in
 - [`../Operations/Configuration.md`](../Operations/Configuration.md) — `appsettings.json` schema and tag list shape.
 - [`../Operations/StatusPage.md`](../Operations/StatusPage.md) — the Kestrel admin endpoint and counter catalog.
 - [`../Reference/LogEvents.md`](../Reference/LogEvents.md) — stable structured log event names.
-- [`../design.md`](../design.md) — canonical design decisions and rationale.
 - [`../Testing/Simulator.md`](../Testing/Simulator.md) — `pymodbus` DL205 simulator used by the end-to-end suite.
-- [`../plan/README.md`](../plan/README.md) — phase plan with per-phase test inventory.

mbproxy/docs/Architecture/ResponseCache.md

@@ -405,5 +405,3 @@ configuration described above.
   `mbproxy.cache.*` event catalogue with event IDs.
 - [`../Testing/Simulator.md`](../Testing/Simulator.md) — the
   `pymodbus` DL205 stand-in used by the end-to-end cache tests.
-- [`../design.md`](../design.md) — canonical design decisions and
-  rationale.