feat(a1): SYS4 container parser (header + code decode + data_array_end shrink)

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

engine/Age.Engine.Tests/Sys4LoaderTests.cs

@@ -0,0 +1,22 @@
+using Age.Engine.Sys4;
+using Xunit;
+
+public class Sys4LoaderTests
+{
+    [Fact]
+    public void ParsesMenuBinLikeSys4load()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var s = Sys4Loader.Load(Path.Combine(Paths.Data1, "MENU.BIN"), table);
+        // header F0..F5 from `sys4load MENU.BIN --summary`
+        Assert.Equal(0x17, s.Header.LocalInt1);
+        Assert.Equal(1, s.Header.LocalFloats);
+        Assert.Equal(1, s.Header.LocalStrings1);
+        Assert.Equal(2, s.Header.LocalInt2);
+        Assert.Equal(1, s.Header.Unknown);
+        Assert.Equal(1, s.Header.LocalStrings2);
+        Assert.Equal(148, s.Instructions.Count);   // sys4load: 148 instructions
+        Assert.Equal(3, s.Strings.Count);           // sys4load: 3 inline strings
+        Assert.All(s.Instructions, ins => Assert.True(s.IndexByOffset.ContainsKey(ins.Offset)));
+    }
+}