feat(a2b): implement 0x208 get-texture-size (geometry keystone)

Promote 0x208 from stub to a real VM op that writes the loaded texture's
width/height into its two output globals, via new IHost.GetTextureSize. This is
the single native primitive the CG-load subroutine (SC0000 label_12649) needs;
all downstream centering/anchor geometry is already computed in bytecode.

Non-Godot hosts return (0,0) so trace/selftest parity holds (engine 9/9 incl.
TraceDiffTests). Godot host gets a temporary (0,0) stub; real impl in the
compositor task.

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

engine/Age.Engine/Hosting/CaptureHost.cs

@@ -11,6 +11,7 @@ public sealed class CaptureHost : IHost
     public void CreateTexture(int slot, int width, int height) { }
     public void SetTexture(long resourceId, int slot) { }
     public void DrawTexture(int slot, int srcX, int srcY, int width, int height, int dstX, int dstY) { }
+    public (int Width, int Height) GetTextureSize(int slot) => (0, 0);
     public void PlayBgm(long id) { }
     public void PlayVoice(long id) { }
 }

engine/Age.Engine/Hosting/IHost.cs

@@ -8,6 +8,7 @@ public interface IHost
     void CreateTexture(int slot, int width, int height);
     void SetTexture(long resourceId, int slot);
     void DrawTexture(int slot, int srcX, int srcY, int width, int height, int dstX, int dstY);
+    (int Width, int Height) GetTextureSize(int slot);
     void PlayBgm(long id);
     void PlayVoice(long id);
 }