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.Tests/TextureGeometryTests.cs

@@ -0,0 +1,46 @@
+using System.Collections.Generic;
+using Age.Engine.Hosting;
+using Age.Engine.Model;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
+using Xunit;
+
+public class TextureGeometryTests
+{
+    private sealed class FakeSizeHost : IHost
+    {
+        public void ShowText(int o, string t) { }
+        public void CallScript(long id) { }
+        public void OnStub(int op) { }
+        public void WaitForInput() { }
+        public void CreateTexture(int slot, int w, int h) { }
+        public void SetTexture(long resId, int slot) { }
+        public void DrawTexture(int slot, int sx, int sy, int w, int h, int dx, int dy) { }
+        public void PlayBgm(long id) { }
+        public void PlayVoice(long id) { }
+        public (int Width, int Height) GetTextureSize(int slot) => (0x140, 0xC8);
+    }
+
+    [Fact]
+    public void GetTextureSizeWritesHostDimsIntoOutputGlobals()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        // 0x208 (global-int 50)(global-int 60)(global-int 61): slot=50, out_w=G[60], out_h=G[61]
+        const int T_GINT = 3;
+        var ins = new Instruction(0, 0x208, new[]
+        {
+            new Operand(T_GINT, 50), new Operand(T_GINT, 60), new Operand(T_GINT, 61),
+        });
+        var script = new Script
+        {
+            Header = new ScriptHeader(0, 0, 0, 0, 0, 0),
+            Instructions = new[] { ins },
+            IndexByOffset = new Dictionary<int, int> { { 0, 0 } },
+            Strings = new Dictionary<int, string>(),
+        };
+        var vm = new VirtualMachine(script, table, new FakeSizeHost());
+        vm.Run();
+        Assert.Equal(0x140, vm.Globals[60]);
+        Assert.Equal(0xC8, vm.Globals[61]);
+    }
+}

engine/Age.Engine.Tests/TextureOpsTests.cs

@@ -18,6 +18,7 @@ public class TextureOpsTests
         public void CreateTexture(int slot, int w, int h) => Creates++;
         public void SetTexture(long resId, int slot) => Sets.Add((resId, slot));
         public void DrawTexture(int slot, int srcX, int srcY, int w, int h, int dstX, int dstY) => Draws.Add((slot, w, h));
+        public (int Width, int Height) GetTextureSize(int slot) => (0, 0);
         public void PlayBgm(long id) { }
         public void PlayVoice(long id) { }
     }

engine/Age.Engine.Tests/WaitForInputTests.cs

@@ -17,6 +17,7 @@ public class WaitForInputTests
         public void CreateTexture(int slot, int w, int h) { }
         public void SetTexture(long resId, int slot) { }
         public void DrawTexture(int slot, int srcX, int srcY, int w, int h, int dstX, int dstY) { }
+        public (int Width, int Height) GetTextureSize(int slot) => (0, 0);
         public void PlayBgm(long id) { }
         public void PlayVoice(long id) { }
     }