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.

Helpers/DragReorderMath.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections;
+
+namespace ytLive.Helpers;
+
+/// <summary>
+/// The arithmetic behind dragging a row in the layer list, kept out of the window's code-behind so it
+/// can be tested without a mouse. It used to live inline in the preview-mouse handlers, and the only
+/// test that covered it injected REAL cursor movement (<c>SetCursorPos</c> + <c>mouse_event</c>) — which
+/// took over whatever machine ran the tests and failed whenever the desktop was busy. 2026-11-05: the
+/// arithmetic moved here (pure, tested), the injected-input test was deleted.
+/// </summary>
+public static class DragReorderMath
+{
+    /// <summary>True once the pointer has travelled far enough to mean "drag", not "click". WPF's
+    /// own thresholds are the caller's (<c>SystemParameters.MinimumHorizontal/VerticalDragDistance</c>).</summary>
+    public static bool ExceededThreshold(double dx, double dy, double horizontal, double vertical)
+        => Math.Abs(dx) >= horizontal || Math.Abs(dy) >= vertical;
+
+    /// <summary>Moves one item to the position currently occupied by another. Returns the index the
+    /// item actually landed on (the caller keeps dragging from there), or -1 when the move is a no-op
+    /// or the indices are out of range.
+    /// <para>Order matters: the item is removed first, so a downward move inserts at the target's old
+    /// index and lands exactly under the pointer — this is the behaviour the mouse test used to pin.</para>
+    /// </summary>
+    public static int TryMove(IList items, int from, int to)
+    {
+        if (items == null || from < 0 || to < 0 || from >= items.Count || to >= items.Count) return -1;
+        if (from == to) return -1;
+
+        var moved = items[from]!;
+        items.RemoveAt(from);
+        items.Insert(to, moved);
+        return to;
+    }
+}