feat(a2b): BmpHeader.ReadDims + gfx diagnostic (headless geometry oracle)

BmpHeader.ReadDims reads texture dims from the pre-converted BMP header (VM-
thread-safe, no pixel decode) — the data source for GetTextureSize. `Age.Cli gfx
<SCENE>` runs a scene and dumps set-texture/get-texture-size/draw-texture in
order with resolved file + computed geometry, mirroring the `audio` diagnostic.

Confirms 0x208 now yields real dims (800x600/800x800/800x227) and sane UI
geometry; surfaces the first-load anchor edge (unseeded state, Phase B).

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

engine/Age.Engine.Tests/BmpHeaderTests.cs

@@ -0,0 +1,33 @@
+using System.IO;
+using Age.Engine.Sys4;
+using Xunit;
+
+public class BmpHeaderTests
+{
+    [Fact]
+    public void ReadDimsReadsWidthAndHeightFromBmpHeader()
+    {
+        // Minimal 54-byte BMP header (BITMAPFILEHEADER 14 + BITMAPINFOHEADER 40); width=4, height=3.
+        var b = new byte[54];
+        b[0] = (byte)'B'; b[1] = (byte)'M';
+        System.BitConverter.GetBytes(40).CopyTo(b, 14);   // header size
+        System.BitConverter.GetBytes(4).CopyTo(b, 18);    // width
+        System.BitConverter.GetBytes(3).CopyTo(b, 22);    // height
+        var tmp = Path.Combine(Path.GetTempPath(), "agehdr_test.bmp");
+        File.WriteAllBytes(tmp, b);
+        try
+        {
+            var (w, h) = BmpHeader.ReadDims(tmp);
+            Assert.Equal(4, w);
+            Assert.Equal(3, h);
+        }
+        finally { File.Delete(tmp); }
+    }
+
+    [Fact]
+    public void ReadDimsReturnsZeroForMissingFile()
+    {
+        var (w, h) = BmpHeader.ReadDims(Path.Combine(Path.GetTempPath(), "does_not_exist_agehdr.bmp"));
+        Assert.Equal((0, 0), (w, h));
+    }
+}