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.

ViewModels/MainViewModel.Sources.cs

@@ -15,14 +15,19 @@ namespace ytLive.ViewModels;
 public partial class MainViewModel
 {
     private void AddSource(object? parameter)
+    {
+        var sourceType = parameter is SourceType typed
+            ? typed
+            : Enum.TryParse<SourceType>(parameter?.ToString(), true, out var parsed) ? parsed : SourceType.Image;
+        AddSource(sourceType);
+    }
+
+    private void AddSource(SourceType sourceType, string? mediaPath = null)
     {
         var scene = StagedScene;
         if (scene == null) return;
         PushPreviewUndo();
 
-        var sourceType = parameter is SourceType typed
-            ? typed
-            : Enum.TryParse<SourceType>(parameter?.ToString(), true, out var parsed) ? parsed : SourceType.Image;
         // Privacy blur is a Live-scene tool: it exists to hide the creator's desktop, and every
         // other scene's art is authored. Rejecting here (not only in the (+) menu) keeps a stale
         // menu item or a scripted call from planting one on BRB.
@@ -40,6 +45,7 @@ public partial class MainViewModel
             SourceType.AlertBox => "Stream Alerts",
             SourceType.Ticker => "Ticker",
             SourceType.Scoreboard => "Scoreboard",
+            SourceType.MediaSource => "Video",
             _ => "Layer",
         };
 
@@ -62,6 +68,7 @@ public partial class MainViewModel
                 ? $"{baseName}-{webCount}"
                 : NextSourceName(scene, baseName),
             Type = sourceType,
+            MediaPath = mediaPath,
             X = sourceType == SourceType.TextOverlay ? 160
                 : sourceType == SourceType.ChatBox ? 1400
                 : sourceType == SourceType.AlertBox ? 620
@@ -79,6 +86,7 @@ public partial class MainViewModel
                 : sourceType == SourceType.AlertBox ? 680
                 : sourceType == SourceType.Scoreboard ? 1920
                 : sourceType == SourceType.Ticker ? 1920
+                : sourceType == SourceType.MediaSource ? 1920
                 : sourceType == SourceType.Blur ? 240 : 0,
             Height = sourceType == SourceType.TextOverlay ? 140
                 : sourceType == SourceType.WebSource ? 480
@@ -86,6 +94,7 @@ public partial class MainViewModel
                 : sourceType == SourceType.AlertBox ? 200
                 : sourceType == SourceType.Scoreboard ? 56
                 : sourceType == SourceType.Ticker ? 48
+                : sourceType == SourceType.MediaSource ? 1080
                 : sourceType == SourceType.Blur ? 160 : 0,
         };
 
@@ -104,6 +113,9 @@ public partial class MainViewModel
             };
         }
 
+        if (sourceType == SourceType.MediaSource)
+            RegisterMediaSource(source);
+
         if (sourceType == SourceType.ChatBox)
         {
             source.PropertyChanged += (_, e) =>
@@ -286,7 +298,18 @@ public partial class MainViewModel
         }
         if (element is Source { Type: SourceType.WebSource } webSource)
             _webView2Manager?.Unregister(webSource.Id);
+
+        // Media: the layer stops listening, and its decode session stops once no scene
+        // references the file any more (sessions are shared per path).
+        string? mediaPath = null;
+        if (element is Source mediaSource && mediaSource.IsMediaSource)
+        {
+            UnregisterMediaSource(mediaSource);
+            mediaPath = mediaSource.MediaPath;
+        }
+
         _sceneGraph.RemoveElement(scene, element);
+        ReleaseMediaIfUnused(mediaPath);
 
         if (element is WebcamSceneConfig)
         {