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/Hosting/IHost.cs

@@ -5,4 +5,7 @@ public interface IHost
     void CallScript(long id);
     void OnStub(int opcode);
     void WaitForInput();
+    void CreateTexture(int slot, int width, int height);
+    void SetTexture(long resourceId, int slot);
+    void DrawTexture(int slot, int x, int y, int width, int height);
 }