fix(textures): DecodeSolidColor null-safe against null ColorValue

A Base1Solid (or OrigTextureId==0) Surface can carry a null ColorValue;
DecodeSolidColor dereferenced it (color.Alpha) and threw NullReferenceException.
It is called directly from TextureCache.DecodeFromDats, OUTSIDE
DecodeRenderSurface's try/catch, so the NRE crashed the whole client. Surfaced
by the D.2b chrome prove-out feeding UI surface ids. Guard null -> Magenta
(the decoder's existing "undecodable" sentinel). Test added.

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

tests/AcDream.Core.Tests/Textures/SurfaceDecoderSolidColorTests.cs

@@ -0,0 +1,17 @@
+using AcDream.Core.Textures;
+using Xunit;
+
+namespace AcDream.Core.Tests.Textures;
+
+public class SurfaceDecoderSolidColorTests
+{
+    [Fact]
+    public void DecodeSolidColor_NullColor_ReturnsMagenta_DoesNotThrow()
+    {
+        // A malformed Base1Solid surface can carry a null ColorValue. DecodeSolidColor
+        // is called outside DecodeRenderSurface's try/catch (from TextureCache), so it
+        // must be null-safe itself — return the undecodable sentinel, never NRE.
+        var result = SurfaceDecoder.DecodeSolidColor(null!, 0f);
+        Assert.Equal(DecodedTexture.Magenta, result);
+    }
+}