Release v3.6: rebuild the self-updater (in-app, rollback-safe); warnings to front

Replace the generated cmd.exe + robocopy update helper — which silently
failed on some machines — with an in-app C# installer:
  * Stage the new version to a per-user temp folder off the install and run
    the new RemSound.exe from there, so nothing in the install is locked by
    the updater itself.
  * Wait for the old process to fully exit (real WaitForExit), then
    back-up-and-swap files in C# with retry + rename-aside; roll the install
    back to the previous version on any failure, so a failed update can never
    leave a half-installed RemSound.
  * Log every step to updater.log; clean up old stages and legacy batch
    artefacts on launch. Removed the old BuildInstallScript batch generator.

Route the "RemSound is already running" dialog and its follow-up message
through ForegroundDialog so they surface in front from a background-relaunched
copy, matching the earlier post-update fix.

Docs: rewrite the readme update sections for the new mechanism, regenerate
MANUAL.md, refresh the About-box changelog and RELEASE_NOTES.

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

src/RemSound.App/AboutDialog.cs

@@ -20,6 +20,20 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v3.6
+
+        Updating is far more reliable. RemSound now installs its
+        own updates, step by step, instead of handing off to a
+        Windows script that could quietly fail. If an update can't
+        finish for any reason, RemSound puts your previous version
+        back exactly as it was and tells you — so a failed update
+        can never leave you stuck or half-installed. Your settings,
+        profiles, logs and sounds are never touched.
+
+        A couple more notices now come to the front too: the
+        "RemSound is already running" message, and the notes shown
+        after an update when RemSound was tucked away in the tray.
+
         RemSound v3.5
 
         Recover a sound card you unplug: if a USB sound card