Compositor: reuse column LUT buffer to avoid per-call allocations (Unit 4)

Services/Compositor/SceneCompositor.cs

@@ -32,6 +32,7 @@ public sealed class SceneCompositor
     private static readonly Dictionary<SceneElement, VideoFrame> _elementRasters = new(ReferenceEqualityComparer.Instance);
     private static readonly object _pasteGate = new();
     private const int MaxPasteEntries = 256;
+    private static float[] _sxLutCache = Array.Empty<float>();
 
     /// <summary>
     /// Composite <paramref name="scene"/> into the tier's output frame. Layer order (back →
@@ -547,7 +548,11 @@ public sealed class SceneCompositor
 
         // Build horizontal LUT for scaled blit to reduce per-pixel divides
         var lutCount = x1 - x0 + 1;
-        var sxLut = new float[lutCount];
+        if (_sxLutCache.Length < lutCount)
+        {
+            _sxLutCache = new float[Math.Max(lutCount, 4096)];
+        }
+        var sxLut = _sxLutCache;
         var pxBase = x0 - ex;
         for (var xi = 0; xi < lutCount; xi++)
         {