feat(a2b): promote create/set/draw-texture to IHost methods (trace parity kept)

Runtime finding: SC0000's bg is a slot-0 full-screen slideshow (res 0x23 first), not the
static-path res 0x21. Target corrected to 0x23. CaptureHost/CountHost no-op the new methods.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

engine/Age.Engine.Tests/WaitForInputTests.cs

@@ -14,6 +14,9 @@ public class WaitForInputTests
         public void CallScript(long id) { }
         public void OnStub(int opcode) { }
         public void WaitForInput() => Waits++;
+        public void CreateTexture(int slot, int w, int h) { }
+        public void SetTexture(long resId, int slot) { }
+        public void DrawTexture(int slot, int x, int y, int w, int h) { }
     }
 
     [Fact]