Add isolated FFmpeg movie decoder shim

godot/MovieRuntime.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Diagnostics;
+using Age.Engine.Sys4;
+
+/// <summary>Presentation-side ownership for one decoder plus its fail-safe completion deadline.</summary>
+internal sealed record MovieRuntime(string Name, int AssetId, IMovieDecoder Decoder,
+                                    long StartedAtTimestamp, long WatchdogMs)
+{
+    public static MovieRuntime Open(string name, int assetId, MoviePayload payload,
+                                    IMovieDecoderFactory factory)
+    {
+        ArgumentNullException.ThrowIfNull(factory);
+        IMovieDecoder decoder = factory.Open(payload);
+        return new MovieRuntime(name, assetId, decoder, Stopwatch.GetTimestamp(),
+            decoder.StopTimeMs is >= 0 and var stopTime
+                ? Math.Clamp(stopTime + 2000, 5000, 300000)
+                : 30000);
+    }
+}