Service: always-on update log + "View service update log" menu item

Local checkpoint - NOT for public release. Ed wants logging for the service updating.

- New ALWAYS-ON update log at ProgramData\RemSound\service\update.log (not gated on the
  service-logging toggle - updates are rare but important). It records the full sequence:
    * "update detected: newer RemSound.exe (5.3) found, running 5.2 - restarting"
    * the restarter's own "stopping" / "service started" (or "START FAILED - <reason>")
    * "update complete: now running version 5.3"
  The restart PowerShell writes its stop/start outcome itself, so the part that runs AFTER
  the old service process is gone - and any failed start - is still captured. A pending
  marker set at detection and consumed on the next start closes the loop (its absence flags
  a stuck update).
- Service menu gains "View service update log" to open it (friendly message if none yet).

Update log + pending-marker round-trip covered by the isolation self-test. Gate 27/27.

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

src/RemSound.App/RemSoundService.cs

@@ -46,6 +46,8 @@ public sealed class RemSoundService : ServiceBase
         // Record the running version + start time so the app's Service menu can show them — this is how a
         // self-update is visible (the version bumps and the start time is recent).
         try { ServiceStore.SaveStatus(new ServiceStore.ServiceStatus { Version = versionText, StartedUtc = DateTime.UtcNow }); } catch { }
+        // If this start is the completion of a self-update restart, close the loop in the update log.
+        try { if (ServiceStore.ConsumeUpdatePending()) ServiceStore.AppendUpdateLog($"update complete: now running version {versionText}"); } catch { }
         host = ServiceSendHost.FromConfig(msg => log?.Event(msg));
         worker = new Thread(() =>
         {
@@ -66,6 +68,11 @@ public sealed class RemSoundService : ServiceBase
         if (restartScheduled) return;
         if (!ServiceUpdate.UpdateLanded()) return;
         restartScheduled = true;
+        var running = ServiceUpdate.RunningVersion();
+        var runningText = running is null ? "?" : $"{running.Major}.{running.Minor}";
+        // Always-on update log (not gated on the service-logging toggle) — updates are rare + important.
+        ServiceStore.AppendUpdateLog($"update detected: newer RemSound.exe ({ServiceUpdate.OnDiskVersion()}) found, running {runningText} — restarting to update");
+        ServiceStore.SetUpdatePending();
         log?.Event("service: a newer RemSound version was installed — restarting to update");
         ServiceUpdate.RestartSelf();
     }