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/AboutDialog.cs

@@ -20,6 +20,14 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v5.2
+
+        A stability and polish release — bug fixes and tidy-ups from a deep code review. No new features; it should just feel a bit more solid.
+
+        Fixed: changing the codec or send rate while streaming could, in rare cases, crash RemSound. Fixed: recording both your sent and received audio into a single file could lose a little audio and drift out of sync over a long recording — it now stays accurate the whole way through. Fixed: after installing, RemSound now stays minimised if that's your setting (and still comes to the front otherwise), and uninstalling one copy no longer switches off another copy's run-at-startup.
+
+        Also: the clipping indicator works again, the router port opened for incoming connections is tidied up when you close, and the app is a little lighter on memory and disk. Nothing about how you connect changed, so it still talks to older versions.
+
         RemSound v5.1
 
         Install RemSound as a proper Windows app.