mbproxy: cross-platform support — Linux/systemd alongside Windows

Make the service build, run, and install on Linux as a first-class
target while keeping the Windows Service + Event Log behaviour intact.

- Build: drop the hardcoded win-x64 RID — single-file publish now works
  for any RID. publish.ps1 gains -Rid; new publish.sh for Linux hosts.
- Diagnostics: DiagnosticSinkSelector picks the Error+ sink per host —
  Windows Event Log under the SCM, local syslog under systemd
  (Serilog.Sinks.SyslogMessages), none for interactive runs. The
  EventLog truncation helper is extracted so it is testable cross-OS.
- Host: Program.cs registers AddSystemd() alongside AddWindowsService().
- Config: a RID-conditioned appsettings template ships Windows or Unix
  paths; both templates are schema-validated by a test.
- Install: systemd unit (Type=exec) plus install.sh / uninstall.sh.
  Also fixes two cross-platform bugs found while testing: install.ps1
  and uninstall.ps1 used New-EventLog / Remove-EventLog (absent in
  PowerShell 7), and the E2E sim launcher hardcoded Windows venv paths.
- Docs updated across README, CLAUDE.md, and docs/ for dual-platform.

413 tests pass on Windows; 374 (all non-simulator) on Linux.

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

mbproxy/src/Mbproxy/Mbproxy.csproj

@@ -12,16 +12,19 @@
     <InformationalVersion>1.0.0</InformationalVersion>
   </PropertyGroup>
 
-  <!-- Single-file self-contained publish (Release only; Debug stays normal for fast iteration).
-       The resulting Mbproxy.exe is ~100 MB because the self-contained publish bundles the full
-       .NET 10 + ASP.NET Core runtime — 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 -p:SelfContained=false -p:PublishSingleFile=true)
-       if the target machine has .NET 10 installed. -->
-  <PropertyGroup Condition="'$(Configuration)' == 'Release'">
+  <!-- Single-file publish settings — apply only to a Release publish with an explicit RID.
+       Publishing with -r <rid> produces a single-file binary, self-contained by default
+       (bundles the .NET 10 + ASP.NET Core runtime, ~100 MB) so no .NET install is needed on
+       the target. Override with -p:SelfContained=false for a framework-dependent build
+       (~1.6 MB) when the target already has the .NET 10 + ASP.NET Core runtime.
+
+       The RID is supplied per publish (win-x64, linux-x64, ...) and is deliberately NOT
+       hardcoded here — see install/publish.ps1 / install/publish.sh. The
+       '$(RuntimeIdentifier)' != '' guard means a plain `dotnet build -c Release` with no RID
+       stays an ordinary framework build (SelfContained without a RID is an SDK error). -->
+  <PropertyGroup Condition="'$(Configuration)' == 'Release' and '$(RuntimeIdentifier)' != ''">
     <PublishSingleFile>true</PublishSingleFile>
     <SelfContained>true</SelfContained>
-    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
     <IncludeNativeLibrariesForSelfExtract>true</IncludeNativeLibrariesForSelfExtract>
   </PropertyGroup>
 
@@ -32,12 +35,19 @@
 
   <ItemGroup>
     <!-- Microsoft.Extensions.Hosting is already included transitively via
-         Microsoft.AspNetCore.App — do not re-add it explicitly. -->
+         Microsoft.AspNetCore.App — do not re-add it explicitly.
+         The two init-system integration packages are both portable: each is
+         safe to reference and call on any OS (the helper self-detects its host
+         and no-ops otherwise), so no conditional reference is needed. -->
     <PackageReference Include="Microsoft.Extensions.Hosting.WindowsServices" Version="10.0.8" />
+    <PackageReference Include="Microsoft.Extensions.Hosting.Systemd" 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" />
+    <!-- Local-syslog sink for the Linux diagnostic bridge (Error+ events).
+         Serilog.Sinks.SyslogMessages is the maintained IonxSolutions package. -->
+    <PackageReference Include="Serilog.Sinks.SyslogMessages" Version="4.1.0" />
     <!-- Polly: backend-connect retry pipeline (PolicyFactory.BuildBackendConnect) and
          listener-recovery pipeline (PolicyFactory.BuildListenerRecovery). -->
     <PackageReference Include="Polly" Version="8.6.6" />
@@ -48,17 +58,29 @@
     <InternalsVisibleTo Include="Mbproxy.Tests" />
   </ItemGroup>
 
+  <!-- Link the platform-appropriate install template as the published appsettings.json so
+       the binary ships with a fully-commented, usable example config (PLCs, BCD tags, all
+       sections present) instead of an empty stub. The .NET configuration loader supports
+       JSONC (comments) under the default Host.CreateApplicationBuilder path, so the comments
+       in the template are valid at runtime.
+
+       The two templates differ only in OS-specific paths (log directory) and platform
+       notes. A `dotnet publish -r linux-*` (or any non-win RID) ships the Linux template;
+       win-* and a plain RID-less dev build ship the Windows one. -->
   <ItemGroup>
-    <!-- Link the install template as the published appsettings.json so the binary ships
-         with a fully-commented, usable example config (one PLC, one BCD tag, all sections
-         present) instead of an empty stub. The .NET configuration loader supports JSONC
-         (comments) under the default Host.CreateApplicationBuilder path, so the comments
-         in the template are valid at runtime. -->
     <None Remove="appsettings.json" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(RuntimeIdentifier)' == '' or $(RuntimeIdentifier.StartsWith('win'))">
     <Content Include="..\..\install\mbproxy.config.template.json"
              Link="appsettings.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
   </ItemGroup>
+  <ItemGroup Condition="'$(RuntimeIdentifier)' != '' and !$(RuntimeIdentifier.StartsWith('win'))">
+    <Content Include="..\..\install\mbproxy.linux.config.template.json"
+             Link="appsettings.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
 
 </Project>