Compositor: parallelize BlitContentRaw row loop (4 threads, BelowNormal intent) to speed scaled blits (Unit 5)

Services/Compositor/SceneCompositor.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Threading.Tasks;
 using ytLive.Helpers;
 using ytLive.Models;
 
@@ -575,10 +576,11 @@ public sealed class SceneCompositor
             sxLut[xi] = sxCanvas;
         }
 
-        for (var y = y0; y <= y1; y++)
+        var parallelOpts = new ParallelOptions { MaxDegreeOfParallelism = 4 };
+        Parallel.For(y0, y1 + 1, parallelOpts, y =>
         {
             var py = y - ey;
-            if (py < oy || py > oy + drawnH) continue;
+            if (py < oy || py > oy + drawnH) return;
 
             // syNorm: normalized [0,1] position within the drawn content (vertically)
             var syNorm = (py - oy) / drawnH;
@@ -656,7 +658,7 @@ public sealed class SceneCompositor
                     }
                 }
             }
-        }
+        });
     }
 
     /// <summary>Two-stage bilinear in 8.8 fixed point; result within ±1 of the