feat(media): add a video file layer that decodes into the scene

TASK 21's picker slice: the decoder, manager, pacing and loop mechanism were
already shipped but nothing ever acquired a session, so a media Source rendered
nothing. Adds "Video File" to the (+) menu (video-filtered OpenFileDialog),
one decode session per distinct path with a HasSession guard, release when the
last scene reference goes, and a post-load reconcile. Loop is a manager-level
per-path answer ("any live reference loops") applied before StartAsync and
updated live via SetLooping. Properties panel shows the file with Change… and
Loop. Volume stays unwired: media audio rides the desktop loopback, TRAX-style.

Reference: OBS's media source (file → decode → composite into the scene) is the
established answer for this feature; the lifecycle shape mirrors this repo's
WebView2/ScreenCapture registration rather than inventing a new one.

Test: ytLive.Tests/MediaSourceAcquisitionTests.cs — add starts one decoder and
frames reach the layer, loop flip reaches the decoder, remove stops it.

Docs: task-21 closed, TASKS row + Day-4 row + stale 3·17 row corrected,
frame-pipeline acquisition note, HANDOFF rewritten. verify.sh green.

Services/MediaVideoSourceManager.cs

@@ -39,6 +39,7 @@ public sealed class MediaVideoSourceManager : IDisposable
 
     private readonly Func<string, IMediaFrameSource?> _sourceFactory;
     private readonly Dispatcher? _uiDispatcher;
+    private readonly Func<string, bool>? _loopProvider;
     private readonly Dictionary<string, MediaSession> _sessions = new();
     private readonly object _gate = new();
 
@@ -48,10 +49,14 @@ public sealed class MediaVideoSourceManager : IDisposable
     /// <summary>Raised when a media decoder cannot be created (bad path).</summary>
     public event Action<string, string>? MediaFailed;
 
-    public MediaVideoSourceManager(Func<string, IMediaFrameSource?> sourceFactory, Dispatcher? uiDispatcher = null)
+    public MediaVideoSourceManager(
+        Func<string, IMediaFrameSource?> sourceFactory,
+        Dispatcher? uiDispatcher = null,
+        Func<string, bool>? loopProvider = null)
     {
         _sourceFactory = sourceFactory;
         _uiDispatcher = uiDispatcher;
+        _loopProvider = loopProvider;
     }
 
     /// <summary>Increments the refcount for a file, starting the decoder the first time.</summary>
@@ -90,6 +95,10 @@ public sealed class MediaVideoSourceManager : IDisposable
                 session = new MediaSession(key, source);
                 session.FrameHandler = frame => OnFrameAvailable(session, frame);
                 session.Source.FrameAvailable += session.FrameHandler;
+                // Loop intent lives on the Source (per scene), so the caller supplies the
+                // per-path answer ("any live reference loops"). Applied before Start so a
+                // zero-length or instantly-EOF file cannot slip past a later assignment.
+                session.Source.Looping = _loopProvider?.Invoke(key) ?? false;
                 _sessions[key] = session;
                 shouldStart = true;
             }
@@ -158,6 +167,32 @@ public sealed class MediaVideoSourceManager : IDisposable
             return _sessions.TryGetValue(key, out var session) ? session.LatestFrame : null;
     }
 
+    /// <summary>True while a decode session exists for the file — the caller uses this to
+    /// keep one acquire per path instead of stacking refcounts on every layout pass.</summary>
+    public bool HasSession(string key)
+    {
+        lock (_gate)
+            return _sessions.ContainsKey(key);
+    }
+
+    /// <summary>The files with a live session, for reconciling a layout load.</summary>
+    public IReadOnlyList<string> GetActiveKeys()
+    {
+        lock (_gate)
+            return new List<string>(_sessions.Keys);
+    }
+
+    /// <summary>Applies a loop change to a running session (the creator flipped the
+    /// toggle). No-op when the file has no session.</summary>
+    public void SetLooping(string key, bool looping)
+    {
+        lock (_gate)
+        {
+            if (_sessions.TryGetValue(key, out var session))
+                session.Source.Looping = looping;
+        }
+    }
+
     /// <summary>The shared preview bitmap for a file (so a config created after the
     /// first frame can adopt the running bitmap), or null.</summary>
     public WriteableBitmap? GetPreviewBitmap(string key)