Fix: add missing System.Collections.Generic import and remove unused var

Also tidies build warnings from Unit 3 changes.

Services/Compositor/SceneCompositor.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using ytLive.Helpers;
 using ytLive.Models;
 
@@ -624,13 +625,7 @@ public sealed class SceneCompositor
             Buffer.BlockCopy(src.BgraPixels, 0, dst, 0, dstW * dstH * 4);
             return;
         }
-        // Fully opaque source, covering entire destination canvas
-        if (op == 256 && src.IsOpaque && x0 == 0 && y0 == 0 && x1 == dstW && y1 == dstH)
-        {
-            Buffer.BlockCopy(src.BgraPixels, 0, dst, 0, dstW * dstH * 4);
-            return;
-        }
-        // Opaque source with 1:1 aligned region covering full canvas? treat same
+        // Opaque source with 1:1 aligned region covering full canvas
         if (op == 256 && src.IsOpaque && ex == 0 && ey == 0 && src.Width == dstW && src.Height == dstH)
         {
             Buffer.BlockCopy(src.BgraPixels, 0, dst, 0, dstW * dstH * 4);

Services/Encoder/FramePump.cs

@@ -344,7 +344,7 @@ public sealed class FramePump : IDisposable
         int stalls = 0;
         long lastCacheHits = 0, lastCacheRenders = 0;
         var statsNext = DateTime.UtcNow + TimeSpan.FromSeconds(5);
-        bool _loggedStartDetail = false;
+        // bool _loggedStartDetail = false;
 
         // Real-render accounting (UNIT 0, 2026-10-05): every average above was
         // divided by statFrames — the number of frames SUBMITTED, which deadline