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/index.md

@@ -8,12 +8,12 @@ Plain data types. No logic beyond what a property can carry. See
 | `Scene.cs` | A scene: `Name`, `IsHidden`, `IsChatScene`, `IsEditing`, **`HasBackdrop`** (Live-only policy flag, default off — only the canonical Live scene has one, see `SceneCatalog`), `Elements` collection (images + webcam config), `WebcamConfig` accessor |
 | `SceneCatalog.cs` | The five canonical scenes (Starting/Live/BRB/Chat/Ending) — the product, by name: `All`, `IsCanonical`, `Is(name, canonical)` (case-insensitive trim), `IsChat`, `HasBackdrop(name)` (true only for Live). Drives the empty-DB seed, the "+" missing-scenes menu, and `MainViewModel.EnforceBackdropPolicy` |
 | `Source.cs` | A source: `SourceType` enum (DisplayCapture/WindowCapture/Background/Image/TextOverlay/ChatBox/Countdown/WebSource/MediaSource/**AlertBox (TASK 43 — the celebration zone, one per layout)** — **Webcam removed**), `X`/`Y`/`Width`/`Height`, `Opacity`, `IsEnabled`, `ImageSource` (static) / `VideoImageSource` (live frames), `DisplaySource` (`IsLiveCapture` → video, else static), `ClipShape` enum (Traditional/Round), `IsMirrored` + `MirrorScale`, `MirrorButtonText`/`ShapeButtonText`, asset identity, `IsImageSource` (XAML binds this, never `Type`), **backdrop surface**: `IsBackdrop` (permanent bottom layer, never removable), `CaptureKey` (persisted `monitor:<n>`/`window:<hwnd>`/`picker:<name>`), `IsLiveCapture` (`Type` is Display/WindowCapture) |
-| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`/`virtual IsZoomLayer` (TASK 52); `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload); **TASK 57:** the geometry setters (`X`/`Y`/`Width`/`Height`) are `virtual` so a derived element can CONSTRAIN every write (the webcam clamps itself into the vertical tier's band) instead of every writer re-implementing the rule |
-| `VerticalCameraLayout.cs` | **TASK 57 (the "vv" feature):** the vertical (9:16) tier's camera rules as pure math — `TileWidth`/`TileHeight` (270×152 master px = 480×270 in the 1080×1920 output), `Inset` (32 master px from the band's outer edge), `IsVerticalTier(QualityOption)` (portrait tier), `AreaFor(band, bottom)` (the band splits into two 607×540 areas), `MaxSizeFor(band)`, `SpotFor(band, w, h, centreX, centreY)` (the six spots — the area and column the tile's **centre** falls in, inset, never outside the band) and `DefaultSpot` (bottom-right, the corner the 16:9 default webcam uses) |
+| `SceneElement.cs` | Base for anything placeable in a scene: shared layout/clip/mirror/border surface, `virtual IsWebcam`/`virtual IsImageSource`/`virtual IsZoomLayer` (TASK 52); `ToggleClipShape` + persisted `RectWidth`/`RectHeight` (pre-Round rect so round→rect restores after reload); **TASK 57:** the geometry setters (`X`/`Y`/`Width`/`Height`) are `virtual` so a derived element can CONSTRAIN every write (the webcam clamps itself into the vertical tier's band) instead of every writer re-implementing the rule; `ClipShape` + `ToggleClipShape()` are `virtual` too, so a derived element can CONSTRAIN the shape (the Chat scene's vertical camera is rect-only) |
+| `VerticalCameraLayout.cs` | **TASK 57 (the "vv" feature):** the vertical (9:16) tier's camera rules as pure math — `TileWidth`/`TileHeight` (270×152 master px = 480×270 in the 1080×1920 output), `Inset` (32 master px from the band's outer edge), `IsVerticalTier(QualityOption)` (portrait tier), `AreaFor(band, bottom)` (the band splits into two 607×540 areas), `MaxSizeFor(band)`, `SpotFor(band, w, h, centreX, centreY)` (the six spots — the area and column the tile's **centre** falls in, inset, never outside the band) and `DefaultSpot` (bottom-right, the corner the 16:9 default webcam uses), `FillsArea(band, width)` (the camera is wide enough to fill a whole area = "enlarged") and `SizeFor(band, requestedWidth)` (the Chat camera's TWO sizes - the small tile or one whole 607×540 area — whichever is nearer the requested width, so the resize handle has two detents) |
 | `ZoomSceneElement.cs` | **Transient zoom box (TASK 52, creator-corrected 2026-10-08):** carries the box rect AND the anchored `CaptureX`/`CaptureY` (the fixed box/2 capture window set on `ActivateZoom`; moving the box never touches it). `IsZoomActive` flips `Kind` Static→Dynamic (leaves the baked base while active); never persisted (LayoutStore save skips non-Source/non-WebcamSceneConfig rows), never shown in the layers list or the element template (`IsZoomLayer` gates both). The compositor SKIPS the active element in every layer loop and draws its 2x content LAST (layer n+1) via `ZoomMagnifier`. |
 | `Webcam.cs` | Singleton webcam identity: `Id`, `DeviceId`, `Name` (one row app-wide) |
 | `BackgroundEffect.cs` | **TASK 51:** per-webcam background effect enum `None` / `Remove` / `Blur`, carried by `WebcamSceneConfig` |
-| `WebcamSceneConfig.cs` | Per-scene webcam placement (subclass of `SceneElement`): geometry + `IsVisible` + border (`BorderColor`/`BorderOpacity`/`BorderWidth`/`BorderAnimation`) + `VideoImageSource`; `WebcamId` links to `Webcam`; **TASK 51:** `BackgroundEffect` + `BackgroundStrength` (0..1, default 0.5) + the radio-pill surface `RemoveBackground`/`BlurBackground` (one on turns the other off; both off = None) + `HasBackgroundEffect` (shows the strength slider); **TASK 57 (the "vv" feature):** the vertical (9:16) tier's OWN placement per scene — `VerticalX/Y/Width/Height/VerticalClipShape/VerticalRectWidth/VerticalRectHeight` (persisted, NULL = no vertical placement yet) + the transient `VerticalBand`/`VerticalLayoutLive`/`LandscapeX…LandscapeRectHeight` (the parked 16:9 layout) + `BeginVerticalLayout`/`EndVerticalLayout`/`CommitLiveLayoutToVertical` (the tier swap) and the `X`/`Y`/`Width`/`Height` overrides that clamp every write into the band |
+| `WebcamSceneConfig.cs` | Per-scene webcam placement (subclass of `SceneElement`): geometry + `IsVisible` + border (`BorderColor`/`BorderOpacity`/`BorderWidth`/`BorderAnimation`) + `VideoImageSource`; `WebcamId` links to `Webcam`; **TASK 51:** `BackgroundEffect` + `BackgroundStrength` (0..1, default 0.5) + the radio-pill surface `RemoveBackground`/`BlurBackground` (one on turns the other off; both off = None) + `HasBackgroundEffect` (shows the strength slider); **TASK 57 (the "vv" feature):** the vertical (9:16) tier's OWN placement per scene — `VerticalX/Y/Width/Height/VerticalClipShape/VerticalRectWidth/VerticalRectHeight` (persisted, NULL = no vertical placement yet) + the transient `VerticalBand`/`VerticalLayoutLive`/`LandscapeX…LandscapeRectHeight` (the parked 16:9 layout) + `BeginVerticalLayout`/`EndVerticalLayout`/`CommitLiveLayoutToVertical` (the tier swap) and the `X`/`Y`/`Width`/`Height` overrides that clamp every write into the band; `VerticalShapeLocked` (the Chat scene's vertical layout is rect-only) **overrides** `ClipShape` + `ToggleClipShape()` so no writer can slip a circle in |
 | `QualityOption.cs` | Resolution tier record `(Display, Fps, Bitrate, Width, Height)`; `Label` formats as `1080p60 (8Mbps)` — drives the bottom-bar dropdown and the output-rect math |
 | `StreamConfig.cs` | `StreamConfig` (note: `TargetBitrate`/`Resolution` defaults are stale — not yet wired to the dropdown), `StreamHealth` (bitrate/FPS/dropped/duration), `StreamStatus` enum |
 | `MicStatus.cs` | Footer mic indicator: `NotConnected` / `Connected` / `Problem` — the state behind the MIC button's status dot (red = no device/capture off, green = capturing, yellow = a requested mic failed: in use/unplugged) |