fix(editor): floorplan reference pass — north convention, cursors, R/T, Set Scale UX (#475)

Fix pass over 2D reference-image (guide) handling plus two adjacent repairs:

- North is now world −Z (FLOORPLAN_VIEW_ROTATION_DEG 90 → 0, mirrored in
  world-grid-snap + mcp README; floorplan-panel de-dupes its local copy).
  A rotation-0 north-up scan reads upright when the compass says aligned,
  and "align north" maps to camera azimuth 0 instead of a 90° jump.
- Guide resize cursor: custom double-arrow cursor aimed at the dragged
  corner in screen space — accounts for image aspect (was atan2(±1,±1),
  square-only), the guide's own rotation, and the floorplan view rotation
  the overlay group renders inside (was ignored entirely).
- R/T rotate a selected reference (guide/scan) in ±45° steps; references
  live in selectedReferenceId, not the viewer selection, so both arms get
  the reference-first branch the Delete arm already had. Locked guides skip.
- 2D-only mode hides the Top View button (drives the display:none 3D
  camera); orbit stays — it spins the synced floorplan view.
- Set Scale UX: locked guides stay clickable (calibration auto-lock made
  them unselectable until reload); starting the flow from 3D switches to
  2D; the length input pre-fills the drawn length in the pre-selected unit
  (imperial pre-filled meters labeled feet); Set Scale flips into Cancel
  while the flow runs (mirrored via referenceScaleActiveGuideId); Hide/
  Clear Scale only render once calibrated; Escape cancels (global arm +
  dialog); Clear Scale and Replace Image drop the calibration auto-lock;
  discoverability: corner-hint row + panel nudge for uncalibrated guides.
- healSceneNodes: strip child refs whose child's parentId names another
  parent (stale reparent leftovers rendered a window twice — duplicate
  React keys in 2D, doubled hosted geometry in 3D) and same-array dupes.
- Editor accepts onLoaderChange so hosts can measure open-to-interactive
  time (community wires it to a PostHog timing event).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/mcp/README.md

@@ -251,7 +251,8 @@ plus the element's own height; slabs additionally carry an absolute
 
 **Heads-up when you compute coordinates outside the editor.** Pascal's
 viewports apply their own rotations on top of the world axes: the 2-D plan
-panel wraps its content in a 90° rotation (`FLOORPLAN_VIEW_ROTATION_DEG`), and
+panel rotates its content by the user's view rotation (north-aligned = 0°,
+`FLOORPLAN_VIEW_ROTATION_DEG` baseline, north = world −Z), and
 the 3-D "top-down" snap preserves the camera's current azimuth, so when invoked
 from the iso default position, world and screen axes are offset by ~45° until
 you orbit to an axis-aligned view. So a layout authored as if