fix(media): a played-out clip can play again; play button beside the Video File label

Reported 2026-11-05: "when I click Loop, the video doesn't play in the preview pane". Two
causes, both real:

- `VideoClipPipe.StartAsync` bailed out once it had run once (`_run != null`), so a clip that
  reached EOF could never be started again — it held its last frame forever.
- Loop is only consulted at end-of-file by a LIVE decoder, so flipping it on a finished clip
  did nothing at all.

Now: the pipe reports running state and is restartable after a natural end; the manager marks a
session finished (keeping its last frame) and exposes `IsRunning`/`RestartAsync`; the layer shows
`IsMediaRunning`/`IsMediaFinished`; flipping Loop on a finished clip restarts it.

UI: `▶ Play` sits to the right of the **Video File** label and is enabled only while Loop is off
(creator ruling: with Loop on the clip already runs continuously, so there is nothing to start).
The panel says "Clip finished — holding the last frame." so a frozen picture explains itself.

Also: verify.sh no longer runs physical-input tests by default. That test moves the real cursor
(SetCursorPos + mouse_event); running it mid-gate hijacks the machine someone is using, which is
exactly how it felt. Opt in with PHYSICAL_TESTS=1 when the desktop is idle; the header documents
the scoped-run command for working hours.

Test: `MediaPlaybackRestartTests` (pipe restart, finished state, Play again, Loop-on-finished).
8 scoped tests green. FULL SUITE NOT RUN — the machine was in use; run scripts/verify.sh in a
quiet window before the next milestone.

Services/MediaVideoSourceManager.cs

@@ -25,6 +25,8 @@ public sealed class MediaVideoSourceManager : IDisposable
         public IMediaFrameSource Source { get; }
         public Action<VideoFrame>? FrameHandler;
         public Action<AudioSample>? AudioHandler;
+        public Action? CompletedHandler;
+        public bool Finished;
         public int RefCount;
         public bool Started;
         public bool AudioMuted;
@@ -112,6 +114,8 @@ public sealed class MediaVideoSourceManager : IDisposable
                 session.Source.FrameAvailable += session.FrameHandler;
                 session.AudioHandler = sample => OnAudioAvailable(session, sample);
                 session.Source.AudioAvailable += session.AudioHandler;
+                session.CompletedHandler = () => OnSessionCompleted(session);
+                session.Source.Completed += session.CompletedHandler;
                 // 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.
@@ -135,6 +139,7 @@ public sealed class MediaVideoSourceManager : IDisposable
                 _sessions.Remove(key);
             session.Source.FrameAvailable -= session.FrameHandler;
             if (session.AudioHandler != null) session.Source.AudioAvailable -= session.AudioHandler;
+            if (session.CompletedHandler != null) session.Source.Completed -= session.CompletedHandler;
             AppLog.Write($"MediaSource: failed to start decoder '{key}': {ex.Message}");
             MediaFailed?.Invoke(key, ex.Message);
             await SafeStopAsync(session.Source);
@@ -156,6 +161,7 @@ public sealed class MediaVideoSourceManager : IDisposable
         if (toStop == null) return;
         toStop.Source.FrameAvailable -= toStop.FrameHandler;
         if (toStop.AudioHandler != null) toStop.Source.AudioAvailable -= toStop.AudioHandler;
+        if (toStop.CompletedHandler != null) toStop.Source.Completed -= toStop.CompletedHandler;
         await SafeStopAsync(toStop.Source);
         toStop.PreviewBitmap = null;
     }
@@ -175,6 +181,7 @@ public sealed class MediaVideoSourceManager : IDisposable
         if (toStop == null) return;
         toStop.Source.FrameAvailable -= toStop.FrameHandler;
         if (toStop.AudioHandler != null) toStop.Source.AudioAvailable -= toStop.AudioHandler;
+        if (toStop.CompletedHandler != null) toStop.Source.Completed -= toStop.CompletedHandler;
         await SafeStopAsync(toStop.Source);
         toStop.PreviewBitmap = null;
     }
@@ -187,6 +194,10 @@ public sealed class MediaVideoSourceManager : IDisposable
             return _sessions.TryGetValue(key, out var session) ? session.LatestFrame : null;
     }
 
+    /// <summary>Raised when a file plays out (natural end, no loop) — the layer is then holding its
+    /// last frame and the creator can start it again.</summary>
+    public event Action<string>? SessionCompleted;
+
     /// <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)
@@ -204,6 +215,39 @@ public sealed class MediaVideoSourceManager : IDisposable
 
     /// <summary>Applies a loop change to a running session (the creator flipped the
     /// toggle). No-op when the file has no session.</summary>
+    /// <summary>False once the file has played to the end. A finished session keeps its last frame
+    /// (and its refcount), so "play again" is a restart, not a re-acquire.</summary>
+    public bool IsRunning(string key)
+    {
+        lock (_gate)
+            return _sessions.TryGetValue(key, out var session) && session.Started && !session.Finished;
+    }
+
+    /// <summary>Restarts a finished file from the top (the Loop switch or "Play again"). The session is
+    /// kept, so the shared preview bitmap and the frame consumers stay valid.</summary>
+    public async Task<bool> RestartAsync(string key)
+    {
+        MediaSession? session;
+        lock (_gate)
+        {
+            if (!_sessions.TryGetValue(key, out session)) return false;
+            session.Finished = false;
+            session.Started = false;
+        }
+
+        try
+        {
+            await session.Source.StartAsync().ConfigureAwait(false);
+            session.Started = true;
+            return true;
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"MediaSource: restarting '{key}' failed: {ex.Message}");
+            return false;
+        }
+    }
+
     public void SetLooping(string key, bool looping)
     {
         lock (_gate)
@@ -226,6 +270,17 @@ public sealed class MediaVideoSourceManager : IDisposable
         }
     }
 
+    private void OnSessionCompleted(MediaSession session)
+    {
+        if (!TryGetActiveSession(session)) return;
+        lock (_gate)
+        {
+            if (session.Source.Looping) return; // it will restart itself
+            session.Finished = true;
+        }
+        SessionCompleted?.Invoke(session.Key);
+    }
+
     private void OnAudioAvailable(MediaSession session, AudioSample sample)
     {
         if (!TryGetActiveSession(session)) return;
@@ -274,6 +329,7 @@ public sealed class MediaVideoSourceManager : IDisposable
         {
             session.Source.FrameAvailable -= session.FrameHandler;
             if (session.AudioHandler != null) session.Source.AudioAvailable -= session.AudioHandler;
+            if (session.CompletedHandler != null) session.Source.Completed -= session.CompletedHandler;
             _ = SafeStopAsync(session.Source);
         }
     }