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.

TASKS.md

@@ -72,7 +72,7 @@
 | 54 | **Marcom round: free-forever positioning + merch storefront** | 🔶 In progress — llamacasty.com copy live 2026-10-08 (pricing stripped); merch POD + app About links queued | [`TASKS/task-54-marcom-free-merch.md`](TASKS/task-54-marcom-free-merch.md) |
 | 55 | **Disk hygiene: log rotation + low-disk guard** | ☐ Queued (2026-10-09) — `startup.log` appears uncapped; also temp alert-clip cleanup + free-space check before recording | [`TASKS/task-55-disk-hygiene.md`](TASKS/task-55-disk-hygiene.md) |
 | 56 | **Pre-publication secret scan (full git history)** | ☐ Queued (2026-10-09) — gate before the Gitea repo goes public; report commit/path only, never values | [`TASKS/task-56-secret-scan.md`](TASKS/task-56-secret-scan.md) |
-| 57 | **Vertical (9:16) tier camera placement — the "vv" feature** | 🔶 **Unit 1 shipped 2026-10-21** (Live scene: own placement per tier — small tile, circle-or-rect, one of six spots; drag-snap + manual entry; 16:9 restored exactly). **Unit 2 open** (Chat scene: rect only, normal or enlarged to one 607×540 area) | [`TASKS/task-57-vertical-camera-layout.md`](TASKS/task-57-vertical-camera-layout.md) |
+| 57 | **Vertical (9:16) tier camera placement — the "vv" feature** | ✅ **Both units shipped 2026-10-21** (Unit 1 = Live scene: own placement per tier — small tile, circle-or-rect, one of six spots; drag-snap + manual entry; 16:9 restored exactly. Unit 2 = Chat scene: rect only — the circle is not offered — the tile or one whole 607×540 area) | [`TASKS/task-57-vertical-camera-layout.md`](TASKS/task-57-vertical-camera-layout.md) |
 
 ---
 
@@ -155,14 +155,16 @@ second encoder path needing a "redirect". Record+simulcast is one ffmpeg with tw
 - **TASK 14** — Branch 2 shipped; branch 3+ in progress
 - **TASK 21** — UI picker slice remaining (Windows-only verification pending)
 - **TASK 32–36** — all queued future work (2026-09-01)
-- **🔶 TASK 57 — the "vv" vertical camera layout.** The vertical tier is a centred band crop of
-  the same master, so the 16:9 camera placement falls outside the cropped window and the creator
-  is not in the vertical stream at all. **Unit 1 shipped 2026-10-21:** each scene keeps its own
-  vertical placement (position, size, shape); the Live scene gets a small tile in one of six spots
-  (top/bottom × left/centre/right), circle or rect, placed by drag-snap or typed into the new
-  Geometry fields; switching back to 16:9 restores the desktop layout exactly. **Unit 2 is next:**
-  the Chat scene in the vertical tier — rect only, either normal or enlarged to one 607×540 area
-  (the creator's table, 2026-10-17). See [`TASKS/task-57-vertical-camera-layout.md`](TASKS/task-57-vertical-camera-layout.md)
+- **✅ TASK 57 — the "vv" vertical camera layout** (both units shipped 2026-10-21). The vertical
+  tier is a centred band crop of the same master, so the 16:9 camera placement falls outside the
+  cropped window and the creator is not in the vertical stream at all. **Unit 1:** each scene keeps
+  its own vertical placement (position, size, shape); the Live scene gets a small tile in one of
+  six spots (top/bottom × left/centre/right), circle or rect, placed by drag-snap or typed into the
+  new Geometry fields; switching back to 16:9 restores the desktop layout exactly. **Unit 2:** the
+  Chat scene in the vertical tier — rect only (the circle option is not offered and the model
+  refuses it), the small tile or enlarged to fill one whole 607×540 area, top or bottom by where the
+  camera is (the creator's table, 2026-10-17). **Left:** the creator's hands-on check of the whole
+  vv feature. See [`TASKS/task-57-vertical-camera-layout.md`](TASKS/task-57-vertical-camera-layout.md)
   and `ai.md` → "Vertical (9:16) tier camera placement".
 - **Webcam resource lifecycle (2026-09-17)** — startup slice shipped (OS poll at start, single-camera
   auto-lock, persistent red alert in the Layers panel on lock failure, re-polls until resolved); gate