fix(ndi): canonicalize 'public' -> 'Public' in discovery + sender group strings (the real bug)

6+ hours of misdiagnosis today, root cause finally found this evening: the user's config.json persisted ndiGroups.discoveryGroups = 'public,teamsiso-input'. NDI group names are case-sensitive in the runtime. Teams broadcasts to the canonical 'Public' (capital P) group. Lowercase 'public' didn't match -> NDI Find returned zero sources forever. NDI Studio Monitor sees Teams sources because it uses default groups (no filter = 'Public'). Every TeamsISO launch that read the config got zero -> looked like a TeamsISO bug.

Fix: add NdiInteropPInvoke.NormalizeGroups that case-folds 'Public' specifically (the most common operator footgun) while passing through custom group names (e.g. 'teamsiso-input') verbatim. Wire it into CreateFinder and CreateSender. End-to-end test: restored bad lowercase config -> launched via Start Menu shortcut -> Serilog now logs 'NDI finder created with groups: Public,teamsiso-input' (note capital P) -> REST returns 2 participants. 264/264 tests passing (Engine 124 +12 NormalizeGroups cases, App 131, Integration 9).

Also adds InternalsVisibleTo on the NdiInterop project so the engine test project can cover the internal helper directly.

src/TeamsISO.Engine.NdiInterop/NdiInteropPInvoke.cs

@@ -35,6 +35,36 @@ public sealed class NdiInteropPInvoke : INdiInterop, IDisposable
         return Marshal.PtrToStringAnsi(ptr) ?? string.Empty;
     }
 
+    /// <summary>
+    /// Normalize a comma-separated NDI group list before handing it to the SDK.
+    /// Returns null if the input is null/whitespace (caller will use the SDK default).
+    ///
+    /// **NDI group names are case-sensitive in the runtime.** "Public" matches; "public"
+    /// does NOT. The default group an unconfigured NDI Sender broadcasts to is "Public"
+    /// (capital P). Operators who type "public" into the discovery groups field then see
+    /// zero sources and report the app as broken — that's how this normalizer came to
+    /// exist (2026-05-16 dev session, ~6h of misdiagnosis). We special-case "public" →
+    /// "Public" to match the most common operator footgun. Other group names are
+    /// passed through verbatim — custom groups like "teamsiso-input" are
+    /// intentionally lowercase and must round-trip unchanged.
+    ///
+    /// Marked internal so the test project can cover the lookup table directly.
+    /// </summary>
+    internal static string? NormalizeGroups(string? groups)
+    {
+        if (string.IsNullOrWhiteSpace(groups)) return null;
+        var parts = groups.Split(',', StringSplitOptions.RemoveEmptyEntries);
+        for (var i = 0; i < parts.Length; i++)
+        {
+            var p = parts[i].Trim();
+            // Canonicalize the standard "Public" group regardless of input casing.
+            if (string.Equals(p, "Public", StringComparison.OrdinalIgnoreCase))
+                p = "Public";
+            parts[i] = p;
+        }
+        return string.Join(",", parts);
+    }
+
     // ---- Discovery ----
 
     public NdiFindHandle CreateFinder(string? groups = null)
@@ -48,7 +78,7 @@ public sealed class NdiInteropPInvoke : INdiInterop, IDisposable
         // same lifetime contract CreateReceiver / CreateSender below have relied on
         // since Phase B-2; if it ever turns out to be wrong, those will fail too. The
         // loopback discovery integration test would catch a regression here.
-        var trimmed = string.IsNullOrWhiteSpace(groups) ? null : groups!.Trim();
+        var trimmed = NormalizeGroups(groups);
         if (trimmed is null)
         {
             var nativeDefault = NdiNative.FindCreateV2(IntPtr.Zero);
@@ -247,7 +277,7 @@ public sealed class NdiInteropPInvoke : INdiInterop, IDisposable
 
     public NdiSenderHandle CreateSender(string outputName, string? groups = null)
     {
-        var trimmedGroups = string.IsNullOrWhiteSpace(groups) ? null : groups!.Trim();
+        var trimmedGroups = NormalizeGroups(groups);
         var nameUtf8 = Marshal.StringToHGlobalAnsi(outputName);
         var groupsUtf8 = trimmedGroups is null
             ? IntPtr.Zero

src/TeamsISO.Engine.NdiInterop/TeamsISO.Engine.NdiInterop.csproj

@@ -4,6 +4,15 @@
     <ProjectReference Include="..\TeamsISO.Engine\TeamsISO.Engine.csproj" />
   </ItemGroup>
 
+  <!-- Grant the engine test project visibility into internals (specifically
+       NdiInteropPInvoke.NormalizeGroups, which gates the "public" vs "Public"
+       NDI group case-folding fix). -->
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
+      <_Parameter1>TeamsISO.Engine.Tests</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>

src/tests/TeamsISO.Engine.Tests/Interop/NdiInteropNormalizeGroupsTests.cs

@@ -0,0 +1,35 @@
+using System.Runtime.Versioning;
+using TeamsISO.Engine.NdiInterop;
+
+namespace TeamsISO.Engine.Tests.Interop;
+
+// NdiInteropPInvoke is marked [SupportedOSPlatform("windows")] because it
+// P/Invokes the Windows-only NDI runtime. The pure NormalizeGroups helper
+// doesn't actually touch native code, but it inherits the platform tag from
+// the enclosing class. Re-declaring SupportedOSPlatform here silences CA1416
+// — these tests still only run on Windows (the Engine.Tests project itself
+// is platform-agnostic but xunit only schedules them when the OS supports).
+[SupportedOSPlatform("windows")]
+
+// NdiInteropPInvoke.NormalizeGroups is internal; the engine tests project has
+// access via InternalsVisibleTo applied to TeamsISO.Engine.NdiInterop.
+public class NdiInteropNormalizeGroupsTests
+{
+    [Theory]
+    [InlineData(null,             null)]
+    [InlineData("",               null)]
+    [InlineData("   ",            null)]
+    [InlineData("Public",         "Public")]   // already canonical
+    [InlineData("public",         "Public")]   // lowercase -> canonical (the bug fix)
+    [InlineData("PUBLIC",         "Public")]   // shouty -> canonical
+    [InlineData("PuBlIc",         "Public")]   // mixed case -> canonical
+    [InlineData("teamsiso-input", "teamsiso-input")] // custom group: pass through
+    [InlineData("Public,teamsiso-input",   "Public,teamsiso-input")]
+    [InlineData("public,teamsiso-input",   "Public,teamsiso-input")] // mixed list normalizes the standard one only
+    [InlineData("teamsiso-input,PUBLIC",   "teamsiso-input,Public")]
+    [InlineData("  public  ,  teamsiso-input  ", "Public,teamsiso-input")] // whitespace trimmed per part
+    public void NormalizeGroups_Maps(string? input, string? expected)
+    {
+        NdiInteropPInvoke.NormalizeGroups(input).Should().Be(expected);
+    }
+}

src/tests/TeamsISO.Engine.Tests/TeamsISO.Engine.Tests.csproj

@@ -26,6 +26,9 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\TeamsISO.Engine\TeamsISO.Engine.csproj" />
+    <!-- Needed by NdiInteropNormalizeGroupsTests to reach the internal
+         NormalizeGroups helper (the "public" → "Public" case-folding fix). -->
+    <ProjectReference Include="..\..\TeamsISO.Engine.NdiInterop\TeamsISO.Engine.NdiInterop.csproj" />
   </ItemGroup>
 
 </Project>