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.

Controls/index.md

@@ -10,9 +10,9 @@ See [`schema.md`](../schema.md) for the memory-map conventions.
 | `BottomBar.xaml(.cs)` | Footer (Row 3): stream stats (bitrate/fps/dropped/duration/health), resolution dropdown (gear menu moved up to TopBar 2026-09-22) |
 | `HealthBanner.xaml(.cs)` | Row 1 status banner + the 7 value→visibility converters live in the app theme, not here |
 | `TopBar.xaml(.cs)` | Row 0 session surface (2026-09-22 redesign): brand + gear menu (left), one segmented REC|ON-AIR mode switch + world actions (Record: Start Recording · Stream: Go Live + Test) + reality line while running (dot/word/elapsed) + End (center), account zone — Sign In or avatar w/ Change Account/Logout context menu (right). `RefreshAvatar()`, `GearButton_Click` |
-| `PreviewPane.xaml(.cs)` | Row 1 Col 1 center: the 1920×1080 `OverlayCanvas`/`SelectionOverlay`/`BrandFlashLayer`/`SocialBarElement`, text pull-out drawer, and the live-controls row (TRAX / game / mic meters + sliders + speakers + socials). **TASK 57:** dragging a webcam under the vertical tier routes through `MainViewModel.MoveWebcamInVerticalTier` (the six-spot snap inside the cropped band) instead of free-floating over the master, and a resize re-snaps the tile onto its spot |
+| `PreviewPane.xaml(.cs)` | Row 1 Col 1 center: the 1920×1080 `OverlayCanvas`/`SelectionOverlay`/`BrandFlashLayer`/`SocialBarElement`, text pull-out drawer, and the live-controls row (TRAX / game / mic meters + sliders + speakers + socials). **TASK 57:** dragging a webcam under the vertical tier routes through `MainViewModel.MoveWebcamInVerticalTier` (the six-spot snap inside the cropped band) instead of free-floating over the master, and a resize re-snaps the tile onto its spot (TASK 57 Unit 2: the resize routes through `MainViewModel.ResizeWebcamInVerticalTier` — the Chat camera's two sizes, then the snap) |
 | `SceneThumbnailStrip.xaml(.cs)` | Row 0 scene-picker strip (`ItemsControl` over `Scenes`), staged-red / live-green border highlight |
-| `LeftPanel.xaml(.cs)` | Row 1 Col 0 left panel: edit mode → layers+properties; live → YouTube chat. Source add/edit/remove, list drag-to-reorder, snapshot revert-accept. **TASK 51:** the webcam properties carry the Background section — `RemoveBackgroundPill` / `BlurBackgroundPill` (`PillToggle`, radio-exclusive via `WebcamSceneConfig.RemoveBackground`/`BlurBackground`) + `BackgroundStrengthSlider` (0..1, 5% snaps, shown only while an effect is on). **TASK 57:** the webcam properties carry a Geometry section (X/Y/W/H) — manual placement of the camera, bound to the **live** placement (the one the current tier shows), gated on `IsWebcamSelected` (not `SelectedElement.IsWebcam` — the TASK 51 test finds that panel by binding text and would match twice) |
+| `LeftPanel.xaml(.cs)` | Row 1 Col 0 left panel: edit mode → layers+properties; live → YouTube chat. Source add/edit/remove, list drag-to-reorder, snapshot revert-accept. **TASK 51:** the webcam properties carry the Background section — `RemoveBackgroundPill` / `BlurBackgroundPill` (`PillToggle`, radio-exclusive via `WebcamSceneConfig.RemoveBackground`/`BlurBackground`) + `BackgroundStrengthSlider` (0..1, 5% snaps, shown only while an effect is on). **TASK 57:** the webcam properties carry a Geometry section (X/Y/W/H) — manual placement of the camera, bound to the **live** placement (the one the current tier shows), gated on `IsWebcamSelected` (not `SelectedElement.IsWebcam` — the TASK 51 test finds that panel by binding text and would match twice); the Shape button's `Visibility` is bound to `CanToggleClipShape`, so the circle option is NOT OFFERED in the vertical Chat layout |
 | `OverlayHost.xaml(.cs)` | Root-grid overlay dialogs: Settings / Report Bug / Feature Request / About (+ licenses + license-key entry). `BackToAbout_Click` lives here |
 | `SceneThumbnailStrip.xaml.cs` | Code-behind for the strip above |
 | `LeftPanel.xaml.cs` | Code-behind for the panel above |