feat(D.5.1): IconComposer — CPU alpha-over icon composite + cache

Adds IconComposer (AcDream.App.UI) which mirrors retail IconData::RenderIcons
(decomp 407524): decodes each RenderSurface layer directly via SurfaceDecoder,
composites them bottom-to-top with Porter-Duff alpha-over, and uploads the
result to a GL texture via TextureCache. Composited handles are keyed by the
(iconId, underlayId, overlayId) tuple so each unique combo is uploaded once.

Adds a public TextureCache.UploadRgba8(byte[], int, int, bool) wrapper — a
thin shell around the existing private overload — so IconComposer can upload
its CPU-side composite without duplicating any GL state logic.

Pure Compose() path is covered by 2 unit tests (opaque top wins; transparent
top preserves bottom). Dat-decode + GL-upload exercised by the visual gate.

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

tests/AcDream.App.Tests/UI/IconComposerTests.cs

@@ -0,0 +1,36 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class IconComposerTests
+{
+    private static byte[] Solid(int w, int h, byte r, byte g, byte b, byte a)
+    {
+        var px = new byte[w * h * 4];
+        for (int i = 0; i < w * h; i++) { px[i*4]=r; px[i*4+1]=g; px[i*4+2]=b; px[i*4+3]=a; }
+        return px;
+    }
+
+    [Fact]
+    public void Compose_alphaOver_topOpaqueLayerWins()
+    {
+        var bottom = (Solid(2, 2, 255, 0, 0, 255), 2, 2); // red, opaque
+        var top    = (Solid(2, 2, 0, 0, 255, 255), 2, 2); // blue, opaque
+        var (rgba, w, h) = IconComposer.Compose(new[] { bottom, top });
+        Assert.Equal(2, w); Assert.Equal(2, h);
+        Assert.Equal(0,   rgba[0]); // R
+        Assert.Equal(0,   rgba[1]); // G
+        Assert.Equal(255, rgba[2]); // B — top layer won
+        Assert.Equal(255, rgba[3]); // A
+    }
+
+    [Fact]
+    public void Compose_alphaOver_transparentTopKeepsBottom()
+    {
+        var bottom = (Solid(1, 1, 255, 0, 0, 255), 1, 1);
+        var top    = (Solid(1, 1, 0, 0, 255, 0), 1, 1); // fully transparent blue
+        var (rgba, _, _) = IconComposer.Compose(new[] { bottom, top });
+        Assert.Equal(255, rgba[0]); // bottom red preserved
+        Assert.Equal(0,   rgba[2]);
+    }
+}