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

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

engine/Age.Engine/Hosting/IScriptProvider.cs

@@ -0,0 +1,10 @@
+using Age.Engine.Model;
+namespace Age.Engine.Hosting;
+
+/// <summary>Resolves a call-script id (a raw SYS4INI file index) to a loaded <see cref="Script"/>.
+/// Implemented in Sys4; injected into the VM so the Vm layer never references Sys4.</summary>
+public interface IScriptProvider
+{
+    /// <summary>The script for this id, or null if the id maps to no known script.</summary>
+    Script? GetById(long id);
+}