fix(engine): guard scaler against zero-dimension source frames

A source frame reporting width or height == 0 (a malformed/glitched NDI
frame, or a sender mid-renegotiation) drove ComputeFitRect into a
division by zero: srcAspect = srcW/srcH with srcH==0 yields Infinity →
NaN width/height, and the Stretch path produced a degenerate copy loop.
Either way a single bad frame could throw out of Scale and bubble up to
the pipeline supervisor as a failure, costing a restart + reconnect.

Now a zero-area source short-circuits to a black, fully-opaque frame at
the target resolution — the same visual the slate path would show — so a
transient bad frame is absorbed silently instead of tearing down the ISO.

src/Dragon-ISO.Engine/Pipeline/ManagedNearestNeighborFrameScaler.cs

@@ -15,6 +15,15 @@ public sealed class ManagedNearestNeighborFrameScaler : IFrameScaler
                 $"ManagedNearestNeighborFrameScaler only supports BGRA input (got {source.Format}). " +
                 "v1.0 receivers request BGRA from NDI; UYVY support comes in v1.5.");
 
+        // Degenerate source guard: a frame reporting a zero (or negative) dimension
+        // would divide-by-zero in ComputeFitRect (srcW/srcH) and produce NaN extents,
+        // or drive a degenerate copy loop on the Stretch path. Rather than throw out
+        // of the hot path and cost the pipeline a supervisor restart, emit a black
+        // opaque frame at the target size — identical to what the no-signal slate
+        // would render — so a single bad frame is absorbed without a reconnect.
+        if (source.Width <= 0 || source.Height <= 0)
+            return BlackFrame(targetWidth, targetHeight, timestampTicks);
+
         var output = new byte[targetWidth * targetHeight * 4];
 
         // Compute the destination rectangle within the target according to aspect mode.
@@ -56,6 +65,14 @@ public sealed class ManagedNearestNeighborFrameScaler : IFrameScaler
         return new ProcessedFrame(targetWidth, targetHeight, timestampTicks, output, PixelFormat.Bgra);
     }
 
+    /// <summary>Allocates a black, fully-opaque BGRA frame at the requested size.</summary>
+    private static ProcessedFrame BlackFrame(int width, int height, long timestampTicks)
+    {
+        var output = new byte[width * height * 4];
+        for (var i = 3; i < output.Length; i += 4) output[i] = 0xFF; // alpha; BGR stay 0
+        return new ProcessedFrame(width, height, timestampTicks, output, PixelFormat.Bgra);
+    }
+
     private static (int X, int Y, int W, int H) ComputeFitRect(int srcW, int srcH, int dstW, int dstH, AspectMode aspect)
     {
         switch (aspect)