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/RemSoundUpdater.cs

@@ -30,7 +30,7 @@ namespace RemSound.App;
 /// single canonical release stream, not to be re-pointed at a fork. If you need to publish
 /// from a different repo, change <see cref="RepoOwner"/> / <see cref="RepoName"/>.
 /// </summary>
-internal sealed class RemSoundUpdater : IDisposable
+internal sealed class RemSoundUpdater
 {
     public const string RepoOwner = "Ednunp";
     public const string RepoName = "RemSound";
@@ -51,11 +51,6 @@ internal sealed class RemSoundUpdater : IDisposable
 
     public string CurrentVersion => Assembly.GetExecutingAssembly().GetName().Version?.ToString(3) ?? "0.0.0";
 
-    public void Dispose()
-    {
-        // HttpClient is static and shared across the process; nothing to dispose here.
-    }
-
     /// <summary>Hit the GitHub Releases API, parse the latest release, return a struct
     /// describing what was found. Returns null if the request fails (network down, rate
     /// limited, repo not found) or if the latest version is not newer than the running
@@ -259,7 +254,7 @@ internal sealed class RemSoundUpdater : IDisposable
             }
 
             Log?.Invoke($"updater: launching in-app installer from {appRoot}, parent PID {pid}");
-            System.Diagnostics.Process.Start(psi);
+            using (System.Diagnostics.Process.Start(psi)) { }
             return true;
         }
         catch (Exception ex)