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/Sys4/BmpHeader.cs

@@ -0,0 +1,25 @@
+namespace Age.Engine.Sys4;
+
+/// <summary>
+/// Reads pixel dimensions from a BMP file header (BITMAPINFOHEADER: width at byte 18, height at byte 22,
+/// both little-endian int32; height may be negative for top-down bitmaps). Used to give the VM the
+/// texture size that opcode 0x208 (get-texture-size) needs, without decoding pixels. Our textures are
+/// pre-converted BMPs (tools/convert_agf.py).
+/// </summary>
+public static class BmpHeader
+{
+    public static (int Width, int Height) ReadDims(string? path)
+    {
+        if (string.IsNullOrEmpty(path) || !File.Exists(path)) return (0, 0);
+        try
+        {
+            var b = new byte[26];
+            using var fs = File.OpenRead(path);
+            if (fs.Read(b, 0, 26) < 26 || b[0] != (byte)'B' || b[1] != (byte)'M') return (0, 0);
+            int w = System.BitConverter.ToInt32(b, 18);
+            int h = System.BitConverter.ToInt32(b, 22);
+            return (System.Math.Abs(w), System.Math.Abs(h));
+        }
+        catch { return (0, 0); }
+    }
+}