feat(a1): .NET 8 solution scaffold + Model + OpcodeTable loader

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

engine/Age.Engine.Tests/OpcodeTableTests.cs

@@ -0,0 +1,18 @@
+using Age.Engine.Sys4;
+using Xunit;
+
+public class OpcodeTableTests
+{
+    [Fact]
+    public void LoadsAll248FromJson()
+    {
+        var t = OpcodeTableJson.Load(Paths.OpcodesJson);
+        Assert.Equal(248, t.Count);
+        Assert.True(t.TryGet(0x55, out var label, out var argc));
+        Assert.Equal("mov", label);
+        Assert.Equal(2, argc);
+        Assert.Equal("u0041BEB0", t.Label(0x90));
+        Assert.Equal(7, t.Argc(0x90));
+        Assert.Equal(-1, t.Argc(0x9999)); // absent -> -1
+    }
+}