fix(preview): draw video layers, unlink Play from Loop, geometry last

Three reports from the creator, plus two bugs the real clip exposed.

1. A video layer was INVISIBLE in the preview. The pane's per-layer Image is Collapsed by
   default and switched on by a whitelist of DataTriggers; MediaSource matched none, so the clip
   decoded into a bitmap that nothing displayed. Same defect the alert box hit on 2026-09-26 and
   the scoreboard after it — now recorded at the top of docs/ai/ui-surface.md with a test that
   asserts the bound Image is Visible (`MediaPreviewDisplayTests`).

2. Play is no longer tied to Loop: the button is always enabled (creator ruling — Loop must not
   toggle Play). Loop still restarts a finished clip, which is the behaviour that was broken.

3. Geometry is now the LAST section of the properties list for every draggable element, with
   opacity at the top; the web source's duplicate geometry block is gone (it now uses the shared
   one, which had been sitting above the media panel).

4. The physical mouse-drag test is deleted. It injected real cursor movement, hijacked whatever
   machine ran it, and failed whenever the desktop was busy. The arithmetic it covered now lives
   in `Helpers/DragReorderMath.cs` — pure, and covered by `DragReorderMathTests` (threshold,
   up/down moves, no-op and out-of-range). The two persistence tests were already synthetic.

5. Real-clip check (creator's `test_greenscreen_moving_subject.mp4`, 1920×1080): the detector
   REJECTED it — a fully saturated green sits at value 1.0 and the detector's 0.98 brightness
   ceiling threw it away. White highlights are excluded by the saturation floor, which is the
   property that actually separates them, so the ceiling is gone. With it fixed, the clip is
   detected as green (high confidence), and keyed through our own pipe 97.8% of pixels go
   transparent. Measured: keying with the canonical #00B140 keys 0.0% of this clip — which is why
   the key colour comes from the measured screen.

Tests: 567 passed, 0 failed, clean build 0 warnings. The gate's scope check flags
`Assets/Videos/test_greenscreen_moving_subject.mp4` — the creator's working copy, deliberately not
committed; the fixture lives under ytLive.Tests/Fixtures.

Models/Source.cs

@@ -256,10 +256,7 @@ public class Source : SceneElement
     public bool MediaIsLooping
     {
         get => _mediaIsLooping;
-        set
-        {
-            if (Set(ref _mediaIsLooping, value)) Raise(nameof(CanPlayMedia));
-        }
+        set => Set(ref _mediaIsLooping, value);
     }
 
     private double _mediaVolume = 1.0;
@@ -293,9 +290,6 @@ public class Source : SceneElement
 
     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