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/AudioReceiver.cs

@@ -241,6 +241,13 @@ public sealed class AudioReceiver : IDisposable
     /// contributions. Resets on read.</summary>
     public int TakeMaxOnPacketMs() => listener.TakeMaxOnPacketMs();
 
+    /// <summary>Worst inter-packet arrival gap (ms) at the user-space UDP socket since the
+    /// last call. Resets on read. Compared with the sender's per-callback gap on the other
+    /// machine, this localises a stall: if the sender's send-callback gap is small but this
+    /// is large, the OS/network between sender and receiver delayed delivery (NIC IRQ
+    /// servicing, scheduler not waking our receive thread, kernel batching). 2026-05-21.</summary>
+    public int TakeMaxInterPacketGapMs() => listener.TakeMaxInterPacketGapMs();
+
     /// <summary>Worst FanOutSource cache-occupancy seen since the last call, expressed in
     /// milliseconds at the mix rate (48 kHz stereo float). With one active render lane the
     /// FanOut should drain to ~0 after every consumer Read; sustained non-zero means a
@@ -299,7 +306,8 @@ public sealed class AudioReceiver : IDisposable
     /// <summary>Take the worst post-decode single-sample step magnitude across all active
     /// stream sessions since the last call, resetting each session's probe. Used by the
     /// diag log to pinpoint where in the pipeline audio discontinuities are being
-    /// introduced.</summary>
+    /// introduced. Returns max-of-(cross, within); for the split values use the XB/WB
+    /// methods below and do NOT also call this in the same drain window.</summary>
     public float TakeMaxPostDecodeStep()
     {
         lock (sessionsLock)
@@ -314,6 +322,38 @@ public sealed class AudioReceiver : IDisposable
         }
     }
 
+    /// <summary>Cross-buffer (packet-boundary) max post-decode step across all sessions.
+    /// Drains each session's cross-buffer counter. 2026-05-21 addition for the click hunt.</summary>
+    public float TakeMaxPostDecodeStepCrossBuffer()
+    {
+        lock (sessionsLock)
+        {
+            var max = 0f;
+            foreach (var s in sessions.Values)
+            {
+                var v = s.TakeMaxPostDecodeStepCrossBuffer();
+                if (v > max) max = v;
+            }
+            return max;
+        }
+    }
+
+    /// <summary>Within-buffer (in-packet content) max post-decode step across all sessions.
+    /// Drains each session's within-buffer counter. 2026-05-21 addition for the click hunt.</summary>
+    public float TakeMaxPostDecodeStepWithinBuffer()
+    {
+        lock (sessionsLock)
+        {
+            var max = 0f;
+            foreach (var s in sessions.Values)
+            {
+                var v = s.TakeMaxPostDecodeStepWithinBuffer();
+                if (v > max) max = v;
+            }
+            return max;
+        }
+    }
+
     public long PcmFrameDiscardedPartials
     {
         get
@@ -395,8 +435,12 @@ public sealed class AudioReceiver : IDisposable
     /// <summary>Take the worst single-sample step out of the ring buffer (after decode +
     /// SessionPlayout.Write, before resampler) since the last call.</summary>
     public float TakeMaxPostRingReadStep() => playoutEngine.TakeMaxPostRingReadStep();
+    public float TakeMaxPostRingReadStepCrossBuffer() => playoutEngine.TakeMaxPostRingReadStepCrossBuffer();
+    public float TakeMaxPostRingReadStepWithinBuffer() => playoutEngine.TakeMaxPostRingReadStepWithinBuffer();
     /// <summary>Take the worst single-sample step out of the resampler since the last call.</summary>
     public float TakeMaxPostResamplerStep() => playoutEngine.TakeMaxPostResamplerStep();
+    public float TakeMaxPostResamplerStepCrossBuffer() => playoutEngine.TakeMaxPostResamplerStepCrossBuffer();
+    public float TakeMaxPostResamplerStepWithinBuffer() => playoutEngine.TakeMaxPostResamplerStepWithinBuffer();
     /// <summary>RingbufferOverflowDropBytes = AggregateDrops minus the deliberate trim+drain
     /// causes. Whatever's left was the producer-side overflow (Write into a full buffer) or
     /// the catastrophic-cap trim from NoteFramesQueued. Both indicate "we genuinely couldn't