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.

Models/Source.cs

@@ -256,7 +256,10 @@ public class Source : SceneElement
     public bool MediaIsLooping
     {
         get => _mediaIsLooping;
-        set => Set(ref _mediaIsLooping, value);
+        set
+        {
+            if (Set(ref _mediaIsLooping, value)) Raise(nameof(CanPlayMedia));
+        }
     }
 
     private double _mediaVolume = 1.0;
@@ -275,6 +278,25 @@ public class Source : SceneElement
         set => Set(ref _mediaMuted, value);
     }
 
+    private bool _isMediaRunning;
+    /// <summary>True while the clip is decoding. False means it has played out and is holding its
+    /// last frame — the panel then offers "Play again" instead of leaving a frozen picture with no
+    /// explanation (the 2026-11-05 "clicking Loop did nothing" report).</summary>
+    public bool IsMediaRunning
+    {
+        get => _isMediaRunning;
+        set
+        {
+            if (Set(ref _isMediaRunning, value)) Raise(nameof(IsMediaFinished));
+        }
+    }
+
+    public bool IsMediaFinished => !IsMediaRunning;
+
+    /// <summary>Play is a manual, one-shot action: while Loop is on the clip restarts itself, so the
+    /// button would be a no-op (creator ruling 2026-11-05 — the button is beside the Video File label).</summary>
+    public bool CanPlayMedia => !MediaIsLooping;
+
     private MediaPlaybackState _mediaPlaybackState = MediaPlaybackState.Stopped;
     public MediaPlaybackState MediaPlaybackState
     {