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.

Models/Source.cs

@@ -118,7 +118,8 @@ public class Source : SceneElement
     /// and sizes it in the preview, so it needs the same selection box, grab and resize handle
     /// (creator ruling 2026-11-01).</summary>
     public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox
-        or SourceType.TextOverlay or SourceType.AlertBox or SourceType.Blur or SourceType.Ticker;
+        or SourceType.TextOverlay or SourceType.AlertBox or SourceType.Blur or SourceType.Ticker
+        or SourceType.MediaSource;
 
     /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
     public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture
@@ -238,10 +239,17 @@ public class Source : SceneElement
         set
         {
             if (Set(ref _mediaPath, value))
+            {
                 Raise(nameof(IsMediaSource));
+                Raise(nameof(MediaFileDisplay));
+            }
         }
     }
 
+    /// <summary>The chosen file's name for the properties panel (path stays in the model).</summary>
+    public string MediaFileDisplay =>
+        string.IsNullOrWhiteSpace(_mediaPath) ? "No video chosen" : System.IO.Path.GetFileName(_mediaPath);
+
     private bool _mediaIsLooping;
     public bool MediaIsLooping
     {