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/Options/MbproxyOptionsBindingTests.cs

@@ -102,6 +102,39 @@ public sealed class MbproxyOptionsBindingTests
         options.Resilience.ListenerRecovery.InitialBackoffMs.ShouldBe([1000, 2000, 5000, 15000, 30000]);
         options.Plcs.ShouldBeEmpty();
         options.BcdTags.Global.ShouldBeEmpty();
+
+        // Keepalive defaults — enabled, with the documented timer values.
+        options.Connection.Keepalive.Enabled.ShouldBeTrue();
+        options.Connection.Keepalive.TcpIdleTimeMs.ShouldBe(30000);
+        options.Connection.Keepalive.TcpProbeIntervalMs.ShouldBe(5000);
+        options.Connection.Keepalive.TcpProbeCount.ShouldBe(4);
+        options.Connection.Keepalive.BackendHeartbeatIdleMs.ShouldBe(30000);
+        options.Connection.Keepalive.BackendHeartbeatProbeAddress.ShouldBe(0);
+    }
+
+    // -------------------------------------------------------------------------
+    // Test 5 — the Connection:Keepalive block binds from configuration
+    // -------------------------------------------------------------------------
+    [Fact]
+    public void MbproxyOptionsBinding_BindsKeepaliveBlock()
+    {
+        var options = BindOptions(new Dictionary<string, string?>
+        {
+            ["Mbproxy:Connection:Keepalive:Enabled"] = "false",
+            ["Mbproxy:Connection:Keepalive:TcpIdleTimeMs"] = "45000",
+            ["Mbproxy:Connection:Keepalive:TcpProbeIntervalMs"] = "7000",
+            ["Mbproxy:Connection:Keepalive:TcpProbeCount"] = "6",
+            ["Mbproxy:Connection:Keepalive:BackendHeartbeatIdleMs"] = "20000",
+            ["Mbproxy:Connection:Keepalive:BackendHeartbeatProbeAddress"] = "1024",
+        });
+
+        var ka = options.Connection.Keepalive;
+        ka.Enabled.ShouldBeFalse();
+        ka.TcpIdleTimeMs.ShouldBe(45000);
+        ka.TcpProbeIntervalMs.ShouldBe(7000);
+        ka.TcpProbeCount.ShouldBe(6);
+        ka.BackendHeartbeatIdleMs.ShouldBe(20000);
+        ka.BackendHeartbeatProbeAddress.ShouldBe(1024);
     }
 
     // -------------------------------------------------------------------------
@@ -129,4 +162,47 @@ public sealed class MbproxyOptionsBindingTests
         result.Failed.ShouldBeTrue("Width=8 should fail schema validation");
         result.Failures.ShouldNotBeEmpty();
     }
+
+    // -------------------------------------------------------------------------
+    // Test 6 — every shipped install template (Windows + Linux) loads as JSONC,
+    //          binds to MbproxyOptions, and passes schema validation. This catches
+    //          a malformed template at build time and keeps the two platform
+    //          variants in lockstep.
+    // -------------------------------------------------------------------------
+    [Theory]
+    [InlineData("mbproxy.config.template.json")]
+    [InlineData("mbproxy.linux.config.template.json")]
+    public void MbproxyOptionsBinding_ShippedInstallTemplate_BindsAndValidates(string templateFileName)
+    {
+        var templatePath = ResolveInstallFile(templateFileName);
+
+        // The templates are JSONC; the .NET JSON config provider skips // and /* */
+        // comments and allows trailing commas, so AddJsonFile loads them directly.
+        var config = new ConfigurationBuilder()
+            .AddJsonFile(templatePath, optional: false)
+            .Build();
+
+        var options = config.GetSection("Mbproxy").Get<MbproxyOptions>() ?? new MbproxyOptions();
+
+        var result = new MbproxyOptionsValidator().Validate(null, options);
+        result.Succeeded.ShouldBeTrue(
+            $"{templateFileName} must pass schema validation — failures: " +
+            string.Join("; ", result.Failures ?? []));
+    }
+
+    /// <summary>
+    /// Resolves an <c>install/</c> file by walking up from the test assembly directory.
+    /// Works from both the Windows dev box and the Linux test box.
+    /// </summary>
+    private static string ResolveInstallFile(string fileName)
+    {
+        for (var dir = new DirectoryInfo(AppContext.BaseDirectory); dir is not null; dir = dir.Parent)
+        {
+            var candidate = Path.Combine(dir.FullName, "install", fileName);
+            if (File.Exists(candidate))
+                return candidate;
+        }
+        throw new FileNotFoundException(
+            $"Could not locate install/{fileName} above {AppContext.BaseDirectory}");
+    }
 }