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/VideoClipPipe.cs

@@ -46,7 +46,7 @@ public sealed class VideoClipPipe : IDisposable
     private readonly bool _audio;
     private readonly ChromaKeyOptions? _key;
     private readonly CancellationTokenSource _cts = new();
-    private IDisposable? _run;
+    private bool _running;
     private IDecodeProcess? _activeVideo;
     private IDecodeProcess? _activeAudio;
 
@@ -72,10 +72,13 @@ public sealed class VideoClipPipe : IDisposable
         _key = chromaKey;
     }
 
+    /// <summary>Runs one pass (or a loop of passes). A pass that ended naturally leaves the pipe
+    /// restartable — the creator can ask a finished clip to play again, which is what the Loop
+    /// switch and the "Play again" button need.</summary>
     public Task StartAsync(CancellationToken cancellationToken = default)
     {
-        if (_run != null) return Task.CompletedTask;
-        _run = _cts.Token.Register(StopInternal);
+        if (_running) return Task.CompletedTask;
+        _running = true;
         _ = Task.Run(RunAsync, CancellationToken.None);
         return Task.CompletedTask;
     }
@@ -83,13 +86,16 @@ public sealed class VideoClipPipe : IDisposable
     public Task StopAsync()
     {
         _cts.Cancel();
-        _run = null;
+        Kill(_activeVideo);
+        Kill(_activeAudio);
+        _running = false;
         return Task.CompletedTask;
     }
 
     public void Dispose()
     {
         _cts.Cancel();
+        _running = false;
         _cts.Dispose();
         _activeVideo?.Dispose();
         _activeAudio?.Dispose();
@@ -97,6 +103,7 @@ public sealed class VideoClipPipe : IDisposable
 
     private void StopInternal()
     {
+        _running = false;
         Kill(_activeVideo);
         Kill(_activeAudio);
     }
@@ -129,6 +136,7 @@ public sealed class VideoClipPipe : IDisposable
         }
         catch (OperationCanceledException)
         {
+            _running = false;
             return;
         }
         catch (Exception ex)
@@ -136,9 +144,11 @@ public sealed class VideoClipPipe : IDisposable
             // A failed decode is not fatal: the alert layer falls back to its animations,
             // and a media layer just holds its last frame.
             AppLog.Write($"Video clip decode failed ({Path.GetFileName(_path)}): {ex.Message}");
+            _running = false;
             return;
         }
 
+        _running = false;
         if (!ct.IsCancellationRequested)
         {
             AppLog.Write($"Video clip end: {Path.GetFileName(_path)} frames={frames} audioChunks={chunks}");