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/install/uninstall.ps1

@@ -122,7 +122,10 @@ if (Test-Path $InstallPath) {
 if ([System.Diagnostics.EventLog]::SourceExists('mbproxy')) {
     Write-Host "Removing Windows Event Log source 'mbproxy'..."
     try {
-        Remove-EventLog -Source 'mbproxy'
+        # .NET API, not Remove-EventLog: the *-EventLog cmdlets exist only in
+        # Windows PowerShell 5.1, not PowerShell 7+. Symmetric with the
+        # SourceExists check above.
+        [System.Diagnostics.EventLog]::DeleteEventSource('mbproxy')
     } catch {
         Write-Warning "Could not remove Event Log source: $_"
     }