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/tests/Mbproxy.Tests/HostSmokeTests.cs

@@ -5,6 +5,8 @@ using Mbproxy.Proxy;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Hosting.Systemd;
+using Microsoft.Extensions.Hosting.WindowsServices;
 using Serilog;
 using Serilog.Core;
 using Serilog.Events;
@@ -71,6 +73,26 @@ public sealed class HostSmokeTests
         // Assert: does not throw / time out.
         await stopTask.ShouldCompleteWithinAsync(TimeSpan.FromSeconds(3));
     }
+
+    [Fact]
+    public async Task HostSmoke_BothInitSystemIntegrations_CoRegister_AndHostRunsCleanly()
+    {
+        // Arrange: register BOTH init-system integrations. Each is a no-op off its
+        // own init system, so on a test run (neither) the default console lifetime
+        // applies — they must co-register without conflict and leave the host
+        // startable and stoppable.
+        var builder = Host.CreateApplicationBuilder();
+        builder.Services.AddWindowsService();
+        builder.Services.AddSystemd();
+        builder.ConfigureForTest(new LoggerConfiguration().CreateLogger());
+
+        using var host = builder.Build();
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+
+        // Act + Assert: start/stop do not throw or time out.
+        await host.StartAsync(cts.Token);
+        await host.StopAsync(cts.Token);
+    }
 }
 
 /// <summary>