Bump to v2.1.0: UPnP, read-only profile lock, sleep/hibernate audio fix

Headline features:
* Automatic router port opening (UPnP / NAT-PMP / PCP). Opt-in via
  Preferences; surfaces external address + carrier-grade NAT detection.
* Lock profile (read-only). New File-menu tick that makes a profile
  load-only — session changes don't persist, no save prompt on close.
  Unblocks unattended shutdowns (NVDA gone, remote dropped, hibernate)
  where the existing save prompt could deadlock.
* Check for updates on startup (default on) + brief countdown notice
  before silent updates install, so a launch-time update doesn't make
  the app silently vanish.
* "Cue sounds" -> "Audio cue sounds" label clarification.

Bug fixes:
* No sound after the computer wakes from sleep. PowerResumeHandler
  rebuilds the audio backend automatically on resume; brief
  "Reconnecting to audio driver" splash during the rebuild.
* Receiver audio silent after waking from hibernate. RefreshAudioDeviceLists
  now treats a transient ASIO probe failure (returns -1/-1 because the
  driver is mid-teardown / mid-reinit) as "retry next tick" instead of
  clearing the user's tick selection.

Diagnostic-only changes (gated on the existing Enable-logs checkbox,
zero cost when off):
* AudioStepProbe split into cross-buffer vs within-buffer maxes so log
  inspection can tell a real-content sharp transient apart from a
  pipeline-boundary glitch. Plumbed through every probe owner.
* New rxNetGapMs + gc0/gc1/gc2 delta columns in the diag log to split
  receive-side jitter into network-layer vs managed-runtime causes.

Files touched: RELEASE_NOTES.md + readme.html + 24 source files across
RemSound.Core / RemSound.Sender / RemSound.Receiver / RemSound.App.
Three new app files: PowerResumeHandler, RouterPortMapper,
UpdateInstallNoticeDialog.

Wire format and audio pipeline unchanged from v1.5 onward — v1.5
through v2.1 peers interoperate.

src/RemSound.Receiver/StreamSession.cs

@@ -53,6 +53,8 @@ internal sealed class StreamSession : IDisposable
     // samples a moment later (after riding through the ring buffer).
     private readonly AudioStepProbe postDecodeStepProbe = new();
     public float TakeMaxPostDecodeStep() => postDecodeStepProbe.TakeMax();
+    public float TakeMaxPostDecodeStepCrossBuffer() => postDecodeStepProbe.TakeMaxCrossBuffer();
+    public float TakeMaxPostDecodeStepWithinBuffer() => postDecodeStepProbe.TakeMaxWithinBuffer();
 
     // === Wire-level sequence tracking (Phase 5, 2026-05-14) ===
     // Every audio packet carries a wire sequence number that monotonically increases per