fix: cold-start discovery + installer shortcuts + single-instance hardening

Three independent fixes bundled because all were chasing the same operator
report: 'I just installed, launched from the shortcut, no participants.'

1) NdiDiscoveryService: poll immediately, then ramp from 200ms to the
   configured interval over the first 3 seconds. PeriodicTimer.WaitForNext-
   TickAsync waits the full interval before its first tick, so for a 500ms
   discovery interval the operator stared at 'no ndi sources yet' for half
   a second on every cold start. Force-poll up front (catches the runtime
   cache), then run a fast inner loop for ~3s while mDNS replies trickle
   in. Both loops share a try/finally so the NDI finder is always disposed.

2) MainViewModel.IsDiscovering: new boolean, true for 8s after engine start
   AS LONG AS no participants have arrived. MainWindow.xaml swaps the
   empty-state copy on this binding:
     IsDiscovering=true  -> 'scanning for ndi sources...' (cyan dot)
     IsDiscovering=false -> 'no ndi sources visible -- is teams in a
                            meeting?' + Refresh CTA
   The old copy ('no ndi sources yet -- open teams and start a meeting')
   was being shown immediately at launch even when discovery just hadn't
   run yet, making the app look broken.

3) App.xaml.cs: single-instance mutex moved from Local\ to Global\. On
   admin-user boxes with UAC disabled, launches from different parents
   (elevated File Explorer, non-elevated shell, etc.) can land in slightly
   different security contexts and a Local\ name can be invisible to the
   sibling. Global\ namespace closes that hole — both processes see the
   same mutex regardless of integrity. Belt-and-braces against future
   dual-instance file/port contention.

4) installer/Package.wxs: add a Desktop shortcut component (per-machine
   feature, HKCU keypath per ICE38/ICE43). Operators who can't find the
   Start Menu entry get the Desktop icon. Both shortcuts target the
   installed exe, NOT a stale path under publish/.

installer/Package.wxs

@@ -40,6 +40,7 @@
     <Feature Id="Main" Title="TeamsISO" Level="1">
       <ComponentGroupRef Id="ApplicationFiles" />
       <ComponentGroupRef Id="Shortcuts" />
+      <ComponentGroupRef Id="DesktopShortcut" />
       <ComponentGroupRef Id="ArpEntry" />
     </Feature>
 
@@ -112,8 +113,9 @@
     </ComponentGroup>
 
     <!--
-        Start Menu shortcut to the WPF host. KeyPath sits on a registry
+        Start Menu shortcut to the WPF host. KeyPath sits on a per-user
-        value so component identity is stable across upgrades.
+        registry value because the .lnk file lives in a per-user Start Menu
+        folder (ICE38/ICE43 enforce this even for perMachine installs).
     -->
     <ComponentGroup Id="Shortcuts" Directory="WildDragonStartMenuFolder">
       <Component Id="StartMenuShortcut" Guid="*">
@@ -135,6 +137,29 @@
       </Component>
     </ComponentGroup>
 
+    <!--
+        Desktop shortcut — belt-and-braces. Operators who lose the Start Menu
+        entry (or whose Start Menu doesn't materialize advertised shortcuts
+        cleanly) still get a Desktop icon. Same HKCU keypath rule as above.
+    -->
+    <StandardDirectory Id="DesktopFolder" />
+    <ComponentGroup Id="DesktopShortcut" Directory="DesktopFolder">
+      <Component Id="DesktopShortcutComponent" Guid="*">
+        <Shortcut Id="DesktopTeamsISO"
+                  Name="TeamsISO"
+                  Description="Per-Participant NDI ISO Controller for Microsoft Teams"
+                  Target="[INSTALLFOLDER]TeamsISO.exe"
+                  WorkingDirectory="INSTALLFOLDER"
+                  Icon="TeamsISOIcon" />
+        <RegistryValue Root="HKCU"
+                       Key="Software\Wild Dragon\TeamsISO"
+                       Name="DesktopShortcut"
+                       Type="integer"
+                       Value="1"
+                       KeyPath="yes" />
+      </Component>
+    </ComponentGroup>
+
     <!--
         ARP icon registry entry. Optional — the MSI auto-fills most ARP
         fields from the Package element. We only need to point at the

src/TeamsISO.App/App.xaml.cs

@@ -27,13 +27,24 @@ namespace TeamsISO.App;
 public partial class App : Application
 {
     /// <summary>
-    /// Per-user mutex name. Including the SID-equivalent (the username) ensures two
+    /// Per-user mutex name. Including the username (acting as a SID proxy) ensures two
     /// different Windows users can each run TeamsISO on the same machine, while one
     /// user can't spawn duplicate instances that would contend over the NDI runtime
     /// and the shared %APPDATA%\TeamsISO\config.json.
+    ///
+    /// The "Global\" prefix puts the named object in the system-wide namespace
+    /// (not session-local or integrity-isolated). This matters because when an
+    /// admin user has UAC effectively disabled, launches from different parents
+    /// (elevated File Explorer, non-elevated shell, etc.) can land in slightly
+    /// different security contexts. A "Local\" mutex was being created in
+    /// different views per integrity level on some boxes, letting two TeamsISO
+    /// instances run concurrently — the second's REST surface couldn't bind port
+    /// 9755 (already held) and its Serilog file sink couldn't open the daily log
+    /// (already held with shared=false), producing a window that looked like
+    /// the app but had no engine attached. Global\ closes that gap.
     /// </summary>
     private static readonly string SingleInstanceMutexName =
-        $"Local\\WildDragon.TeamsISO.SingleInstance.{Environment.UserName}";
+        $"Global\\WildDragon.TeamsISO.SingleInstance.{Environment.UserName}";
 
     private System.Threading.Mutex? _singleInstanceMutex;
     private bool _ownsSingleInstanceMutex;

src/TeamsISO.App/MainWindow.xaml

@@ -689,11 +689,40 @@
                 <!-- Empty-state placeholder. Renders when no NDI participants
                      have been discovered yet. Mono sentence + one tertiary
                      Refresh button — no illustration, no mascot, per the v2
-                     shape brief's empty-states section. -->
+                     shape brief's empty-states section.
+
+                     Two visual flavors gated by IsDiscovering (the VM holds
+                     it true for ~8s after engine start, false thereafter):
+                       - IsDiscovering=true  → "Scanning for NDI sources…"
+                                               (neutral; cold-start can take
+                                               1-3s for mDNS to settle)
+                       - IsDiscovering=false → the explanatory empty state
+                                               ("open teams and start a
+                                               meeting") + Refresh CTA
+                     This stops operators from staring at a "broken-looking"
+                     empty table during the first second of every launch. -->
                 <StackPanel HorizontalAlignment="Center"
                             VerticalAlignment="Center"
                             Visibility="{Binding ParticipantCount, Converter={StaticResource CountToVis}, ConverterParameter=empty}">
-                    <TextBlock Text="no ndi sources yet — open teams and start a meeting"
+                    <!-- Discovering: cyan dot + neutral progress copy. -->
+                    <StackPanel Orientation="Horizontal"
+                                HorizontalAlignment="Center"
+                                Visibility="{Binding IsDiscovering, Converter={StaticResource BoolToVis}}">
+                        <Ellipse Width="7" Height="7"
+                                 Fill="{DynamicResource Wd.Accent.Cyan}"
+                                 VerticalAlignment="Center"
+                                 Margin="0,0,10,0"/>
+                        <TextBlock Text="scanning for ndi sources…"
+                                   FontFamily="{StaticResource Wd.Font.Mono}"
+                                   FontSize="12"
+                                   Foreground="{DynamicResource Wd.Text.Secondary}"
+                                   VerticalAlignment="Center"/>
+                    </StackPanel>
+                    <!-- Not discovering (grace window expired with no sources):
+                         the explanatory empty state. -->
+                    <StackPanel HorizontalAlignment="Center"
+                                Visibility="{Binding IsDiscovering, Converter={StaticResource BoolToVisInverse}}">
+                        <TextBlock Text="no ndi sources visible — is teams in a meeting?"
                                    FontFamily="{StaticResource Wd.Font.Mono}"
                                    FontSize="12"
                                    Foreground="{DynamicResource Wd.Text.Tertiary}"
@@ -708,6 +737,7 @@
                                 FontSize="11"
                                 ToolTip="Rebuild the NDI finder"/>
                     </StackPanel>
+                </StackPanel>
                 </Grid>
             </Border>
         </Grid>

src/TeamsISO.App/ViewModels/MainViewModel.cs

@@ -31,6 +31,19 @@ public sealed partial class MainViewModel : ObservableObject, IDisposable
     private readonly Dictionary<Guid, ParticipantViewModel> _byId = new();
     private string _statusText = "Starting…";
 
+    /// <summary>
+    /// Wall-clock at which <see cref="InitializeAsync"/> kicked the engine. Used to
+    /// gate the "Scanning for NDI sources…" placeholder so it shows for a few
+    /// seconds after launch even when ParticipantCount == 0 (the bleak
+    /// "no ndi sources yet" empty state was being shown immediately and
+    /// operators assumed the app was broken before discovery had a chance to fire).
+    /// Null until InitializeAsync runs.
+    /// </summary>
+    private DateTimeOffset? _engineStartedAt;
+
+    /// <summary>How long after engine start to keep showing "Scanning…" instead of the empty-state copy.</summary>
+    private static readonly TimeSpan DiscoveryGracePeriod = TimeSpan.FromSeconds(8);
+
     // _pendingPresetName / Deadline / Applied + the auto-apply path
     // moved to MainViewModel.PresetCommands.cs.
 
@@ -233,6 +246,21 @@ public sealed partial class MainViewModel : ObservableObject, IDisposable
     }
     private int _participantCount;
 
+    /// <summary>
+    /// True for the first <see cref="DiscoveryGracePeriod"/> after engine start.
+    /// The XAML uses this to swap the empty-state placeholder from the bleak
+    /// "no ndi sources yet — open teams and start a meeting" copy (which reads
+    /// as broken to operators who just launched into an active meeting) to a
+    /// neutral "Scanning for NDI sources…" status while NDI Find resolves
+    /// mDNS responses. Always false once participants populate.
+    /// </summary>
+    public bool IsDiscovering
+    {
+        get => _isDiscovering;
+        private set => SetField(ref _isDiscovering, value);
+    }
+    private bool _isDiscovering;
+
     /// <summary>
     /// Currently-enabled (live) ISO count — feeds the v2 transport strip's
     /// "LIVE N" readout. The number is cyan-tinted when non-zero to draw
@@ -530,6 +558,19 @@ public sealed partial class MainViewModel : ObservableObject, IDisposable
         ParticipantCount = totalParticipants;
         LiveCount = enabledCount;
 
+        // IsDiscovering gates the "Scanning for NDI sources…" placeholder.
+        // True for DiscoveryGracePeriod after engine start AS LONG AS we
+        // haven't seen any participants yet; once anything arrives we drop
+        // out of the discovering state immediately (back to the OK path).
+        if (totalParticipants == 0 && _engineStartedAt is { } startedAt)
+        {
+            IsDiscovering = DateTimeOffset.UtcNow - startedAt < DiscoveryGracePeriod;
+        }
+        else if (IsDiscovering)
+        {
+            IsDiscovering = false;
+        }
+
         // Session timer — start on first ISO going live, reset when none are
         // live anymore. Subsequent enables after a full-zero gap restart the
         // timer rather than resuming, which is the operator's mental model:
@@ -600,6 +641,8 @@ public sealed partial class MainViewModel : ObservableObject, IDisposable
     public async Task InitializeAsync(CancellationToken cancellationToken)
     {
         StatusText = "Discovering NDI sources…";
+        _engineStartedAt = DateTimeOffset.UtcNow;
+        IsDiscovering = true;
         await _controller.StartAsync(cancellationToken);
         StatusText = $"Engine running at {_controller.GlobalSettings.FramerateHz:F2} fps target.";
 

src/TeamsISO.Engine/Discovery/NdiDiscoveryService.cs

@@ -77,6 +77,57 @@ public sealed class NdiDiscoveryService
     /// <summary>Long-running poll loop. Cancel the token to stop.</summary>
     public async Task RunAsync(TimeSpan pollInterval, CancellationToken cancellationToken)
     {
+        // Wrap the whole method in try/finally so _finder is always disposed,
+        // including the cancellation paths inside the fast-poll loop.
+        try
+        {
+            // First poll happens IMMEDIATELY — PeriodicTimer.WaitForNextTickAsync
+            // waits the full interval before its first tick, which created a
+            // noticeable cold-start window where the UI showed "no ndi sources yet"
+            // for ~500ms (or whatever the interval is) before discovery had a chance
+            // to fire. Operators launching into a meeting that was already broadcasting
+            // saw an empty table and assumed it was broken. Poll once up front to
+            // pull whatever the NDI runtime has already cached, then settle into
+            // the regular poll cadence.
+            try { PollOnce(); }
+            catch (Exception ex) { _logger.LogWarning(ex, "Initial discovery poll failed; falling through to timer loop."); }
+
+            // Aggressive poll cadence for the first ~3 seconds so cold-start mDNS
+            // resolution surfaces quickly (mDNS responses can lag the initial socket
+            // setup by 200-1500ms depending on the network and the responder); after
+            // that ramp down to the operator-configured pollInterval.
+            var fastInterval = TimeSpan.FromMilliseconds(200);
+            var startedAt = DateTimeOffset.UtcNow;
+            var rampToFullAfter = TimeSpan.FromSeconds(3);
+
+            using (var fastTimer = new PeriodicTimer(fastInterval))
+            {
+                try
+                {
+                    while (DateTimeOffset.UtcNow - startedAt < rampToFullAfter)
+                    {
+                        if (!await fastTimer.WaitForNextTickAsync(cancellationToken)) break;
+                        if (Interlocked.Exchange(ref _refreshRequested, 0) == 1)
+                        {
+                            try
+                            {
+                                _logger.LogInformation("Rebuilding NDI finder on operator request.");
+                                _finder.Dispose();
+                                _finder = _interop.CreateFinder(_discoveryGroups);
+                                _previous.Clear();
+                            }
+                            catch (Exception ex)
+                            {
+                                _logger.LogWarning(ex, "Finder refresh failed; continuing with existing finder.");
+                            }
+                        }
+                        try { PollOnce(); }
+                        catch (Exception ex) { _logger.LogWarning(ex, "Discovery poll failed; will retry on next tick."); }
+                    }
+                }
+                catch (OperationCanceledException) { return; }
+            }
+
             using var timer = new PeriodicTimer(pollInterval);
             try
             {
@@ -101,6 +152,7 @@ public sealed class NdiDiscoveryService
                 }
             }
             catch (OperationCanceledException) { /* expected */ }
+        }
         finally
         {
             _finder.Dispose();