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/Diagnostics/DiagnosticSinkSelector.cs

@@ -0,0 +1,60 @@
+namespace Mbproxy.Diagnostics;
+
+/// <summary>
+/// The platform diagnostic sink to wire for <c>Error</c>+ events — picked once,
+/// at the composition root, by <see cref="DiagnosticSinkSelector"/>.
+/// </summary>
+internal enum DiagnosticSink
+{
+    /// <summary>
+    /// No platform diagnostic sink — console (and rolling-file) sinks only. Used
+    /// for interactive / dev runs on every OS.
+    /// </summary>
+    None,
+
+    /// <summary>
+    /// Windows Application Event Log, via <see cref="EventLogBridge"/>. Selected
+    /// only when the process is hosted as a Windows Service.
+    /// </summary>
+    EventLog,
+
+    /// <summary>
+    /// Local syslog, via <see cref="SyslogBridge"/>. Selected only when the
+    /// process is hosted as a systemd service on Linux.
+    /// </summary>
+    Syslog,
+}
+
+/// <summary>
+/// Pure platform-selection logic for the <c>Error</c>+ diagnostic sink. Holds no
+/// I/O and no host APIs so it is unit-testable for every OS / host combination;
+/// the host detection itself happens in <see cref="HostingExtensions.AddMbproxySerilog"/>.
+/// </summary>
+internal static class DiagnosticSinkSelector
+{
+    /// <summary>
+    /// Picks the diagnostic sink for the current host:
+    /// <list type="bullet">
+    ///   <item>Windows hosted as a Windows Service → <see cref="DiagnosticSink.EventLog"/>.</item>
+    ///   <item>Linux hosted as a systemd service → <see cref="DiagnosticSink.Syslog"/>.</item>
+    ///   <item>Everything else — interactive / dev runs, macOS, launches not owned
+    ///         by an init system → <see cref="DiagnosticSink.None"/>.</item>
+    /// </list>
+    /// The managed-service gate mirrors the original <see cref="EventLogBridge"/>
+    /// contract: a diagnostic sink is wired only when an init system actually owns
+    /// the process, so dev / console runs never need an Event Log source registered
+    /// or a syslog socket reachable.
+    /// </summary>
+    /// <param name="isWindows">Running on Windows.</param>
+    /// <param name="isWindowsService">Hosted by the Windows Service Control Manager.</param>
+    /// <param name="isSystemd">Hosted by systemd.</param>
+    public static DiagnosticSink Select(bool isWindows, bool isWindowsService, bool isSystemd)
+    {
+        // Windows takes precedence: isSystemd is meaningless there, and on
+        // non-Windows isWindowsService is always false.
+        if (isWindows)
+            return isWindowsService ? DiagnosticSink.EventLog : DiagnosticSink.None;
+
+        return isSystemd ? DiagnosticSink.Syslog : DiagnosticSink.None;
+    }
+}