Service: one-click Update-service-to-this-version (no uninstall/reinstall)

Because the service now runs from its own admin-only copy, getting a new build into it
meant uninstall+reinstall (two UAC prompts). Add a DoUpdate path + Service-menu item
that does it in one: stop -> overwrite the ProgramData bin copy from the running app ->
start. Wired as the --update-service verb (elevated), recognised by the service-verb
gate. Enabled only when the service is installed; falls back to a plain install if not.

Note: the service bin folder stays admin-only writable on purpose (a user-writable
SYSTEM binary would be a privilege-escalation hole), so the copy is done elevated via
the one UAC prompt rather than in-process.

Gate: 40/40.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/SelfTest.cs

@@ -1399,7 +1399,7 @@ internal static class SelfTest
         foreach (var verb in new[]
                  {
                      ServiceControl.RunVerb, ServiceControl.InstallVerb, ServiceControl.UninstallVerb,
-                     ServiceControl.StartVerb, ServiceControl.StopVerb,
+                     ServiceControl.UpdateVerb, ServiceControl.StartVerb, ServiceControl.StopVerb,
                  })
         {
             Check(Program.IsServiceInvocation(new[] { verb }), $"'{verb}' must be recognised as a service invocation");
@@ -1417,7 +1417,7 @@ internal static class SelfTest
         if (!loadedBefore)
             Check(!IsAssemblyLoaded(svcAsm), "deciding a normal launch must not load the Windows-service assembly");
 
-        return "normal launches stay load-safe; all five service verbs recognised (case-insensitive)";
+        return "normal launches stay load-safe; all six service verbs recognised (case-insensitive)";
     }
 
     /// <summary>The Service menu's "View service log" opens the newest diagnostic log — the log that says