vv webcam Unit 2: the Chat scene in the vertical tier - rect only, tile or one whole 607x540 area

The creator's locked table (2026-10-17) gives the Chat scene its own rules in
the 9:16 tier: RECT ONLY - the circle option is not offered - and a size that
is either the normal small tile or ENLARGED to fill one whole 607x540 area
(top or bottom). Unit 1 shipped the per-scene placement; this unit is the Chat
scene's own variant of it.

The rule lives on the MODEL, not on the button:
- Models/SceneElement.cs: ClipShape and ToggleClipShape() are now virtual.
- Models/WebcamSceneConfig.cs: VerticalShapeLocked (set per scene by the VM)
  OVERRIDES the ClipShape property, so no writer can slip a circle in - the
  toggle, the Shape button and the placement swap all go through it. The lock
  is cleared BEFORE the 16:9 layout comes back, so the desktop shape returns.
- ViewModels/MainViewModel.Webcam.cs: CanToggleClipShape (the circle is NOT
  offered in the vertical Chat layout - the Shape button disappears, it is not
  merely greyed out), IsChatSceneCamera, ResizeWebcamInVerticalTier.
- Models/VerticalCameraLayout.cs: FillsArea (the "enlarged" state, derived -
  no new column) and SizeFor - the Chat camera's TWO sizes, the tile or the
  whole area, whichever is nearer the requested width, so the resize handle
  has two detents instead of any size in between.
- An enlarged camera FILLS the area its centre is in (top or bottom), so
  dragging it up/down swaps the areas.
- The Chat camera is auto-tiled like the Live one when the vertical tier is
  picked - otherwise the vertical chat stream has no camera at all.
- Controls/PreviewPane.xaml.cs: the resize routes through the VM; the drag
  assigns, lets the model clamp into the band, then snaps (so the enlarged
  camera can be dragged from the top area to the bottom one).

Test: ytLive.Tests/VerticalChatCameraLayoutTests.cs - the ONE Good Dog test for
the unit: the vertical tier is picked -> the Chat camera comes into the band as
a small RECT tile -> the circle option is not offered and cannot be set even on
the model -> dragged up and enlarged it fills the TOP area exactly -> back to
16:9 the desktop layout is restored exactly and the circle option is offered
again. 430/430 tests pass, 0 warnings.

Research (the creator's table is the established practice, not our invention):
- Streamlabs' vertical guide: "splitting the screen in half horizontally,
  placing the camera feed in the top half and the main content on the bottom
  (or vice versa) may be the bet", and a just-chatting vertical wants the
  camera "to take up most of the screen"
  (streamlabs.com/content-hub/post/overlays-vertical-and-horizontal-streaming)
- Bloopbot's vertical overlay docs describe the same per-layer model we use at
  scene granularity: "move or resize a layer on Vertical and it stops being
  Auto-placed. It keeps the place you gave it, and the horizontal layout is not
  touched" (bloopbot.com/docs/vertical-overlays-twitch-dual-format) - our
  swap-at-tier-change, stored per scene.
- Landmine recorded: "about the bottom 560 px of the vertical video can be
  covered by Twitch's chat and player controls on phones" (Bloopbot) - the Chat
  camera's enlarged BOTTOM area may be half-hidden by the platform chrome;
  TASK 53's safe-zone work decides that.

Memory in the same commit: ai.md (the Chat rules, the shape lock, the tests),
TASKS.md + TASKS/task-57-vertical-camera-layout.md (TASK 57 complete), HANDOFF.md,
MyMistakes.md (3 recipes: lock the model property not the UI button; two detents
for a discrete size set; the edit tool cannot match an oldString with an em
dash), Models/ViewModels/Controls index rows.

Models/VerticalCameraLayout.cs

@@ -38,6 +38,22 @@ public static class VerticalCameraLayout
     public static (double Width, double Height) MaxSizeFor(Rect band)
         => (band.Width, band.Height / 2);
 
+    /// <summary>True when a camera is wide enough to fill one whole area of the
+    /// band — the "enlarged" Chat layout (one 607×540 area).</summary>
+    public static bool FillsArea(Rect band, double width) => width >= band.Width - 1;
+
+    /// <summary>The two sizes a camera may take in the vertical tier's Chat
+    /// layout: the small tile (normal) or one whole area (enlarged). Returns
+    /// the one nearer the requested width, so the resize handle snaps between
+    /// them instead of taking any size in between.</summary>
+    public static (double Width, double Height) SizeFor(Rect band, double requestedWidth)
+    {
+        var (areaWidth, areaHeight) = MaxSizeFor(band);
+        return Math.Abs(requestedWidth - TileWidth) <= Math.Abs(requestedWidth - areaWidth)
+            ? (TileWidth, TileHeight)
+            : (areaWidth, areaHeight);
+    }
+
     /// <summary>
     /// The spot a tile snaps to: the area (top/bottom) and the column
     /// (left/centre/right) its centre falls in, then the tile's top-left for