v5.2: stability and polish — deep-audit bug fixes + install-flow fixes

Verified findings from a multi-dimension code audit, plus the two install-flow bugs:
- Fix Opus encoder use-after-free on a codec/rate change while streaming (guard swap vs encode).
- Fix "both" single-file recording dropping audio + drifting (drain both directions in lockstep).
- Fix broken clip counter, UPnP teardown on exit, auto-update-restart foreground grant, and a
  malformed-Opus-format packet orphaning a playout session forever.
- Post-install relaunch now respects start-minimised; uninstall is path-aware so it won't clear a
  different copy's run-at-startup.
- Perf/hygiene: cache AppConfig off UI hot paths, fold per-peer EQ+gain into one pass, deterministic
  disposal (tray menu, timers, COM shortcut, Process handles, process meter), ring-buffer overflow
  guard, receiver session-lock fix, remote-control allow-list moved onto the UI thread.
- Remove dead code (two IsAsioBackend, SessionPlayout.Reset, IsSameEndpoint, RemSoundUpdater
  IDisposable); several stale-doc fixes.

Deferred (not in this release): drift-estimator tweak, peer-discovery pruning, uninstall retry-loop,
encryption nonce. Wire format unchanged (interops v3.3-v5.1). Version -> 5.2.

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

src/RemSound.App/RouterPortMapper.cs

@@ -226,9 +226,12 @@ internal sealed class RouterPortMapper : IDisposable
 
     public void Dispose()
     {
-        if (disposed) return;
-        disposed = true;
+        // Run the full Stop() teardown FIRST (remove the router mapping, stop discovery, unsubscribe the
+        // static NatUtility.DeviceFound handler) while disposed is still false — otherwise Stop()'s own
+        // `if (disposed) return;` guard would skip all of it, leaving the forwarded port open until its
+        // lease expires and the object subscribed to the process-wide event. THEN mark disposed.
         try { Stop(); } catch { /* shutting down */ }
+        lock (gate) { disposed = true; }
     }
 
     private void OnDeviceFound(object? sender, DeviceEventArgs args)