mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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

mbproxy/src/Mbproxy/Proxy/Cache/CacheLogEvents.cs

@@ -2,7 +2,7 @@ namespace Mbproxy.Proxy.Cache;
 
 /// <summary>
 /// Source-generated <see cref="LoggerMessage"/> definitions for the response cache.
-/// Event names are stable — do not rename without updating <c>docs/design.md</c>'s
+/// Event names are stable — do not rename without updating <c>docs/Reference/LogEvents.md</c>'s
 /// Logging event-name table.
 ///
 /// <para>Levels are conservative — a busy PLC under steady cache pressure would emit one

mbproxy/src/Mbproxy/Proxy/Multiplexing/CoalescingLogEvents.cs

@@ -2,7 +2,7 @@ namespace Mbproxy.Proxy.Multiplexing;
 
 /// <summary>
 /// Source-generated <see cref="LoggerMessage"/> definitions for the read-coalescing
-/// feature. Event names are stable — do not rename without updating docs/design.md's
+/// feature. Event names are stable — do not rename without updating docs/Reference/LogEvents.md's
 /// "Logging" event-name table.
 ///
 /// <para>Levels are intentionally conservative — coalescing fires on every overlapping

mbproxy/src/Mbproxy/Proxy/Multiplexing/MultiplexerLogEvents.cs

@@ -3,7 +3,7 @@ namespace Mbproxy.Proxy.Multiplexing;
 /// <summary>
 /// Source-generated <see cref="LoggerMessage"/> definitions for the TxId-multiplexing
 /// connection layer. Event names are stable — do not rename without updating
-/// docs/design.md's "Logging" event-name table.
+/// docs/Reference/LogEvents.md's "Logging" event-name table.
 /// </summary>
 internal static partial class MultiplexerLogEvents
 {

mbproxy/src/Mbproxy/Proxy/Multiplexing/PlcMultiplexer.cs

@@ -420,7 +420,7 @@ internal sealed class PlcMultiplexer : IAsyncDisposable, IMultiplexCountersProvi
             {
                 // Close every attached pipe that had a request in flight; the others will
                 // simply re-issue on next request through a fresh backend connect.
-                // Per the design doc, ALL attached upstreams cascade on backend disconnect.
+                // Per docs/Architecture/ConnectionModel.md, ALL attached upstreams cascade on backend disconnect.
                 upstreamCount = _pipes.Count;
 
                 // Snapshot keys before disposal modifies the dictionary indirectly.

mbproxy/src/Mbproxy/Proxy/ProxyCounters.cs

@@ -4,7 +4,7 @@ namespace Mbproxy.Proxy;
 /// Immutable snapshot of per-PLC counters. Consumed by the status page.
 /// All fields are point-in-time reads; no ordering guarantees across fields.
 ///
-/// <para><b>Backwards-compat policy (see docs/kpi.md):</b> fields are <i>added</i>, never
+/// <para><b>Backwards-compat policy:</b> fields are <i>added</i>, never
 /// renamed or removed.</para>
 /// </summary>
 public sealed record CounterSnapshot(
@@ -95,7 +95,7 @@ public sealed record CounterSnapshot(
     /// <summary>
     /// Count of coalesced response fan-outs that were skipped because the attached
     /// upstream pipe had already disconnected. A spike is a churn indicator; the metric
-    /// itself is informational (Tier 2 in <c>docs/kpi.md</c>).
+    /// itself is informational.
     /// </summary>
     long CoalescedResponseToDeadUpstream,
     /// <summary>

mbproxy/src/Mbproxy/Proxy/ProxyWorker.cs

@@ -14,7 +14,7 @@ namespace Mbproxy.Proxy;
 /// <summary>
 /// <see cref="BackgroundService"/> that owns all <see cref="PlcListenerSupervisor"/> instances.
 ///
-/// Startup posture (matches design doc "eager, continue on per-port failure"):
+/// Startup posture (matches docs/Architecture/Overview.md "eager, continue on per-port failure"):
 /// <list type="number">
 ///   <item>Enumerate <see cref="MbproxyOptions.Plcs"/> and build one supervisor per PLC.</item>
 ///   <item>Start all supervisors in parallel. Each supervisor attempts to bind immediately

mbproxy/src/Mbproxy/Proxy/RewriterLogEvents.cs

@@ -2,7 +2,7 @@ namespace Mbproxy.Proxy;
 
 /// <summary>
 /// Source-generated <see cref="LoggerMessage"/> definitions for the BCD rewriter pipeline.
-/// All event names are stable — do not rename without updating docs/design.md.
+/// All event names are stable — do not rename without updating docs/Reference/LogEvents.md.
 /// </summary>
 internal static partial class RewriterLogEvents
 {