feat(iso): the 2.5D floor is the Flat plane, one wall-top rise for tiles (#713)

- Vertical oblique projection: the floor matrix is the identity and a height
  rises straight up by z·sin 20°, so the on-screen wall height is unchanged
  and the cos 20° foreshortening of the plan, decor and anchors is gone.
- Device tiles and lock badges stand on the wall-top plane with one common
  shift; the #651 placement search no longer runs in the live scene (its
  removal is #714). Room names keep their Flat floor point.
- The 2.5D fit no longer reserves the 48 CSS px nudge budget.
- Switching projection keeps the camera when the previous projection was on
  screen: saving the setting, entering an editor from 2.5D and adopting a warm
  memo from the other projection re-read only the scalar zoom. A cold 2.5D
  start still opens the 2.5D home.
- Opening faces are ordered along the oblique projector (s·y + z).

Witness: demo/smoke_iso_flat_parity.mjs (AC2–AC5, AC11) is red on the old code.

Issue: #713
User-Visible: yes
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

src/live-editor.ts

@@ -64,7 +64,6 @@ interface LiveEditorHost {
   /** #521: guides for the gesture in flight; `nothing` while the runtime loads. */
   _renderAlignGuides: () => unknown;
   _livePos: (device: { id: string }) => { x: number; y: number };
-  _scenePoint: (point: number[]) => number[];
   _renderProjection: string;
   _spaceModel: () => SpaceModel | null;
   _spaceWalls: WallEntry[];
@@ -309,11 +308,11 @@ const paintDevice = (host: LiveEditorHost, root: ParentNode): void => {
   const element = [...root.querySelectorAll<HTMLElement>('[data-hp="device"]')]
     .find((candidate) => candidate.dataset.id === drag.id);
   if (!device || !element) return;
+  // #713: the 2.5D floor is the Flat plane, and editors are Flat anyway.
   const position = host._livePos(device);
-  const point = host._scenePoint([position.x, position.y]);
   const view = host._viewOr(host._baseVb());
-  element.style.left = `${((point[0] - view.x) / view.w) * 100}%`;
-  element.style.top = `${((point[1] - view.y) / view.h) * 100}%`;
+  element.style.left = `${((position.x - view.x) / view.w) * 100}%`;
+  element.style.top = `${((position.y - view.y) / view.h) * 100}%`;
 };
 
 const makeDecorShapeTransparent = (