mbproxy: initial commit through Phase 9 (TxId multiplexing)

Adds the mbproxy service end-to-end. Phases 00-08 implement the
production-ready single-listener / 1:1-backend transparent Modbus TCP
proxy with bidirectional BCD rewriting for the ~54-PLC DL205/DL260
fleet. Phase 9 replaces the connection layer with a single backend
socket per PLC plus MBAP TxId rewriting, lifting the H2-ECOM100's
4-concurrent-client cap as an operational ceiling.

Phase 9 additions of note:
- PlcMultiplexer + UpstreamPipe + TxIdAllocator + CorrelationMap
- InFlightRequest with IReadOnlyList<InterestedParty> (load-bearing
  for Phase 10 read coalescing — do not collapse to a single field)
- Per-request watchdog: surfaces Modbus exception 0x0B to upstream
  on BackendRequestTimeoutMs, defending against lost responses,
  dead-PLC paths, and pymodbus 3.13.0's concurrent-multiplexed-
  request bug (its ServerRequestHandler.last_pdu state race)
- Status DTO + HTML gain inFlight / maxInFlight / txIdWraps /
  disconnectCascades / queueDepth (Tier 1.6 in docs/kpi.md)

Tests: 263 unit + 38 E2E. Multiplexer correctness under truly
concurrent backend traffic is proved against a stub backend in
PlcMultiplexerTests; MultiplexerE2ETests paces requests so pymodbus
3.13's single-PDU framer stays in known-good mode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/src/Mbproxy/Mbproxy.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Worker">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <OutputType>Exe</OutputType>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <RootNamespace>Mbproxy</RootNamespace>
+    <AssemblyName>Mbproxy</AssemblyName>
+    <!-- Phase 08: Assembly version.  CI can override via /p:InformationalVersion=... -->
+    <InformationalVersion>1.0.0</InformationalVersion>
+  </PropertyGroup>
+
+  <!-- Phase 08: single-file self-contained publish (Release only; Debug stays normal for fast iteration).
+       NOTE: the resulting Mbproxy.exe is ~100 MB because the self-contained publish bundles the full
+       .NET 10 + ASP.NET Core runtime. This exceeds the original 50 MB target in the phase spec;
+       the runtime size is a fixed cost of self-contained deployment on .NET 10 with ASP.NET Core.
+       Operators who need a smaller footprint can use a framework-dependent publish
+       (dotnet publish -c Release -r win-x64 - -self-contained false /p:PublishSingleFile=true)
+       if the target machine has .NET 10 installed. -->
+  <PropertyGroup Condition="'$(Configuration)' == 'Release'">
+    <PublishSingleFile>true</PublishSingleFile>
+    <SelfContained>true</SelfContained>
+    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
+    <IncludeNativeLibrariesForSelfExtract>true</IncludeNativeLibrariesForSelfExtract>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- ASP.NET Core for the Phase 07 Kestrel-hosted admin endpoint. -->
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <!-- Microsoft.Extensions.Hosting is already included transitively via
+         Microsoft.AspNetCore.App — do not re-add it explicitly. -->
+    <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices" Version="10.0.8" />
+    <PackageReference Include="Serilog.Extensions.Hosting" Version="10.0.0" />
+    <PackageReference Include="Serilog.Settings.Configuration" Version="10.0.0" />
+    <PackageReference Include="Serilog.Sinks.Console" Version="6.1.1" />
+    <PackageReference Include="Serilog.Sinks.File" Version="7.0.0" />
+    <!-- Referenced now so phase 04/05 don't need to touch this csproj; usage is deferred -->
+    <PackageReference Include="Polly" Version="8.6.6" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <!-- Allow test project to access internal types (HeartbeatWorker, HostingExtensions, etc.) -->
+    <InternalsVisibleTo Include="Mbproxy.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Update="appsettings.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
+</Project>