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/CheckSoundService.cs

@@ -16,6 +16,11 @@ internal static class CheckSoundService
 {
     private static CuePlayer? checkSound;
     private static CuePlayer? uncheckSound;
+    // Cached at Reload() so Play() (which fires on the UI thread for every genuine checkbox toggle)
+    // doesn't re-read + re-deserialize the whole config file from disk just to fetch one bool. Reload()
+    // is already the hook that runs whenever cue settings change, so these stay current.
+    private static bool enableCheckboxOn;
+    private static bool enableCheckboxOff;
 
     /// <summary>When true, <see cref="Play"/> is a no-op. MainForm sets this around bulk programmatic
     /// control updates (profile load, "uncheck all", device-list refresh). The per-call Focused gate
@@ -29,6 +34,8 @@ internal static class CheckSoundService
     public static void Reload()
     {
         var cfg = AppConfig.Load();
+        enableCheckboxOn = cfg.EnableCheckboxOnCue;
+        enableCheckboxOff = cfg.EnableCheckboxOffCue;
         checkSound = LoadCue(MainForm.CueId.CheckboxOn, "check.wav", cfg);
         uncheckSound = LoadCue(MainForm.CueId.CheckboxOff, "uncheck.wav", cfg);
     }
@@ -36,9 +43,8 @@ internal static class CheckSoundService
     public static void Play(bool isChecked)
     {
         if (Suppressed) return;
-        var cfg = AppConfig.Load();
-        if (isChecked) { if (cfg.EnableCheckboxOnCue) checkSound?.Play(); }
-        else { if (cfg.EnableCheckboxOffCue) uncheckSound?.Play(); }
+        if (isChecked) { if (enableCheckboxOn) checkSound?.Play(); }
+        else { if (enableCheckboxOff) uncheckSound?.Play(); }
     }
 
     private static CuePlayer? LoadCue(string cueId, string defaultFile, AppConfig cfg)