The viewport was jumping unexpectedly when placing the first room on a
blank map due to two interacting issues:

1. The mapOrigin (center of bounds used for coordinate centering) was
   recalculated every time map content changed. When the first room was
   added, mapOrigin shifted from (0,0) to the room's center, causing all
   render coordinates to shift while the view offset stayed the same.

2. The initial scale effect's guard ref was not set for blank maps, so
   when mapBounds transitioned from null to valid, auto-zoom triggered
   unexpectedly.

Fixed by:
- Making mapOrigin stable within a map session (only updates when the
  map file changes, not when content is edited)
- Setting the initial scale guard for blank maps immediately, preventing
  auto-zoom when first geometry is added

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>