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/install.ps1

@@ -165,7 +165,10 @@ if (-not (Test-Path $configDest)) {
 
 if (-not [System.Diagnostics.EventLog]::SourceExists('mbproxy')) {
     Write-Host "Registering Windows Event Log source 'mbproxy'..."
-    New-EventLog -Source 'mbproxy' -LogName 'Application'
+    # .NET API, not New-EventLog: the *-EventLog cmdlets exist only in Windows
+    # PowerShell 5.1, not PowerShell 7+. This call is symmetric with the
+    # SourceExists check above and works on every PowerShell edition.
+    [System.Diagnostics.EventLog]::CreateEventSource('mbproxy', 'Application')
 } else {
     Write-Host "Windows Event Log source 'mbproxy' already registered."
 }