feat(editor): 2D plan proximity + equal-spacing guides for openings

Route the door/window floor-plan placement dimensions through the new
opening-guides service:
  - edge-to-edge clearance to the nearest neighbour (or wall end) on each
    side, now with overlap suppression (previously nearest-only, ad-hoc).
  - Figma-style equal-spacing — a "=" badge per gap on the wall centreline
    whenever the moving opening is part of a run of 3+ (near-)equally-spaced
    openings.

Adds the `equal-spacing-badge` FloorplanGeometry primitive, its 2D renderer
(distinct pink accent), and overlay registration. Shown while placing/moving.
Sill height + vertical alignment are 3D-only (a top-down plan has no vertical
axis) and land in the next phase.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/registry/types.ts

@@ -440,6 +440,20 @@ export type FloorplanGeometry =
       /** Rotation in radians. The renderer auto-flips to keep text upright. */
       angle: number
     }
+  /**
+   * Equal-spacing badge — a small accent pill marking one gap in a run of
+   * (near-)equally-spaced openings (the 2D counterpart of Figma's "=" distance
+   * chips). Emitted once per equal gap so the repeated value reads as a rhythm.
+   * `text` is the shared gap distance; `angle` orients the pill along the wall
+   * (the renderer auto-flips it upright).
+   */
+  | {
+      kind: 'equal-spacing-badge'
+      point: FloorplanPoint
+      text: string
+      /** Rotation in radians. */
+      angle: number
+    }
   /**
    * Architect's dimension overlay — extension lines from the edge
    * endpoints out past the dimension line, two dimension line halves