Implement SC0000 movie playback lifecycle

engine/Age.Engine/Model/GfxState.cs

@@ -185,6 +185,16 @@ public sealed class GfxState
         lock (_lock)
             return _objects.TryGetValue(handle, out var o) ? o.SourceSlot : -1;
     }
+
+    /// <summary>Rebind one retained object from one source surface to another. Used by SC0000's bounded
+    /// movie site to reproduce the native warm-engine slot assignment before the following static loaders
+    /// reuse the port's cold-bootstrap slot.</summary>
+    public void RemapObjectSurface(long handle, int fromSlot, int toSlot)
+    {
+        lock (_lock)
+            if (_objects.TryGetValue(handle, out var obj) && obj.SourceSlot == fromSlot)
+                obj.SourceSlot = toSlot;
+    }
     public bool IsRegistered(long handle) { lock (_lock) { return _operandRegistry.Contains(handle); } }
     public long QueryField(long idx) => _fieldTable.TryGetValue(idx, out var v) ? v : 0;