Add IScriptProvider + Sys4ScriptProvider (call-script id -> Script)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/Sys4ScriptProviderTests.cs

@@ -0,0 +1,20 @@
+using Age.Engine.Sys4;
+using Xunit;
+
+public class Sys4ScriptProviderTests
+{
+    [Fact]
+    public void ResolvesKnownIdsToTheirScripts()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var provider = Sys4ScriptProvider.Load(table);
+
+        var additem = provider.GetById(0x1ab);   // ADDITEM.BIN
+        var mes = provider.GetById(0x2ae7);      // MES.BIN
+        Assert.NotNull(additem);
+        Assert.NotNull(mes);
+        Assert.True(additem!.Instructions.Count > 0);
+        Assert.Same(additem, provider.GetById(0x1ab));   // cached: same instance
+        Assert.Null(provider.GetById(long.MaxValue));    // unknown id
+    }
+}